Update CONTRIBUTING.md

Add link for rewriting Git history in preparation for Derek's new
commit message linting feature.

CONTRIBUTING.md

-## Contributing
+# Contributing
 
-### Guidelines
+## Guidelines
 
 Guidelines for contributing.
 
-#### How can I get involved?
+### How can I get involved?
 
 First of all, we'd love to welcome you into our Slack community where we exchange ideas, ask questions and chat about OpenFaaS, Raspberry Pi and other cloud-native technology. (*See below for how to join*)
 
@@ -24,11 +24,11 @@ We have a number of areas where we can accept contributions:
 
 This is just a short list of ideas, if you have other ideas for contributing please make a suggestion.
 
-#### I've found a typo
+### I've found a typo
 
 * A Pull Request is not necessary. Raise an [Issue](https://github.com/openfaas/faas/issues) and we'll fix it as soon as we can. 
 
-#### I have a [great] idea
+### I have a (great) idea
 
 The OpenFaaS maintainers would like to make OpenFaaS the best it can be and welcome new contributions that align with the project's goals. Our time is limited so we'd like to make sure we agree on the proposed work before you spend time doing it. Saying "no" is hard which is why we'd rather say "yes" ahead of time. You need to raise a proposal.
 
@@ -46,7 +46,7 @@ What makes a good proposal?
 
 If you are proposing a new tool or service please do due diligence. Does this tool already exist? Can we reuse it? For example: a timer / CRON-type scheduler for invoking functions. 
 
-#### Paperwork for Pull Requests
+### Paperwork for Pull Requests
 
 Please read this whole guide and make sure you agree to our DCO agreement (included below):
 
@@ -57,7 +57,7 @@ Please read this whole guide and make sure you agree to our DCO agreement (inclu
 * Always give instructions for testing
 * Provide us CLI commands and output or screenshots where you can
 
-**Commit messages**
+### Commit messages
 
 The first line of the commit message is the *subject*, this should be followed by a blank line and then a message describing the intent and purpose of the commit.
 
@@ -80,6 +80,8 @@ duties.
 Signed-off-by: Alex Ellis <alex@openfaas.com>
 ```
 
+If you would like to ammend your commit follow this guide: [Git: Rewriting History](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History)
+
 **Unit testing with Golang**
 
 Please follow style guide on [this blog post](https://blog.alexellis.io/golang-writing-unit-tests/) from [The Go Programming Language](https://www.amazon.co.uk/Programming-Language-Addison-Wesley-Professional-Computing/dp/0134190440)
@@ -92,8 +94,7 @@ Please raise an Issue or email alex@openfaas.com for an invitation to our Slack
 
 We use vendoring for projects written in Go. This means that we will maintain a copy of the source-code of dependencies within Git. It allows a repeatable build and isolates change. 
 
-* Legacy: `vndr` - https://github.com/LK4D4/vndr
-* New projects: `dep` - https://github.com/golang/dep
+We use Golang's `dep` tool to manage dependencies for Golang projects - https://github.com/golang/dep
 
 **How do I become a maintainer?**
 
@@ -114,29 +115,33 @@ If you have been added to the MAINTAINERS file in the root of an OpenFaaS reposi
 
 OpenFaaS is an independent project created by Alex Ellis which is now being built by a growing community of contributors.
 
-### Community
+## Branding guidelines
+
+For press, branding, logos and marks see the [OpenFaaS media repository](https://github.com/openfaas/media).
+
+## Community
 
 This project is written in Golang but many of the community contributions so far have been through blogging, speaking engagements, helping to test and drive the backlog of FaaS. If you'd like to help in any way then that would be more than welcome whatever your level of experience.
 
-#### Community file
+### Community file
 
 The [community.md](https://github.com/openfaas/faas/blob/master/community.md) file highlights blogs, talks and code repos with example FaaS functions and usages. Please send a Pull Request if you are doing something cool with FaaS.
 
-#### Roadmap
+### Roadmap
 
 Checkout the [roadmap](https://github.com/openfaas/faas/blob/master/ROADMAP.md) and [open issues](https://github.com/openfaas/faas/issues).
 
-#### Slack
+### Slack
 
 There is an Slack community which you are welcome to join to discuss FaaS, IoT and Raspberry Pi projects. Ping [Alex Ellis](https://github.com/alexellis) with your email address so that an invite can be sent out.
 
-Email: alex@openfaas.com - please send in a one-liner about yourself so we can give you a warm welcome and help you get started.
+Please send in a short one-line message about yourself to alex@openfaas.com so that we can give you a warm welcome and help you get started.
 
-### License
+## License
 
 This project is licensed under the MIT License.
 
-#### Copyright notice
+### Copyright notice
 
 Please add a Copyright notice to new files you add where this is not already present:
 
@@ -145,7 +150,7 @@ Please add a Copyright notice to new files you add where this is not already pre
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 ```
 
-#### Sign your work
+### Sign your work
 
 > Note: all of the commits in your PR/Patch must be signed-off.
 
@@ -203,3 +208,5 @@ If you set your `user.name` and `user.email` git configs, you can sign your
 commit automatically with `git commit -s`.
 
 * Please sign your commits with `git commit -s` so that commits are traceable.
+
+If you forgot to sign your work and want to fix that, see the following guide: [Git: Rewriting History](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History)