jkbradley I put the elastic net under the **Algorithm guide** section. Also add the formula of elastic net in mllib-linear `mllib-linear-methods#regularizers`.

dbtsai I left the code tab for you to add example code. Do you think it is the right place?

Author: Shuo Xiang <shuoxiangpub@gmail.com>

Closes #6504 from coderxiang/elasticnet and squashes the following commits:

f6061ee [Shuo Xiang] typo
90a7c88 [Shuo Xiang] Merge remote-tracking branch 'upstream/master' into elasticnet
0610a36 [Shuo Xiang] move out the elastic net to ml-linear-methods
8747190 [Shuo Xiang] merge master
706d3f7 [Shuo Xiang] add python code
9bc2b4c [Shuo Xiang] typo
db32a60 [Shuo Xiang] java code sample
aab3b3a [Shuo Xiang] Merge remote-tracking branch 'upstream/master' into elasticnet
a0dae07 [Shuo Xiang] simplify code
d8616fd [Shuo Xiang] Update the definition of elastic net. Add scala code; Mention Lasso and Ridge
df5bd14 [Shuo Xiang] use wikipeida page in ml-linear-methods.md
78d9366 [Shuo Xiang] address comments
8ce37c2 [Shuo Xiang] Merge branch 'elasticnet' of github.com:coderxiang/spark into elasticnet
8f24848 [Shuo Xiang] Merge branch 'elastic-net-doc' of github.com:coderxiang/spark into elastic-net-doc
998d766 [Shuo Xiang] Merge branch 'elastic-net-doc' of github.com:coderxiang/spark into elastic-net-doc
89f10e4 [Shuo Xiang] Merge remote-tracking branch 'upstream/master' into elastic-net-doc
9262a72 [Shuo Xiang] update
7e07d12 [Shuo Xiang] update
b32f21a [Shuo Xiang] add doc for elastic net in sparkml
937eef1 [Shuo Xiang] Merge remote-tracking branch 'upstream/master' into elastic-net-doc
180b496 [Shuo Xiang] Merge remote-tracking branch 'upstream/master'
aa0717d [Shuo Xiang] Merge remote-tracking branch 'upstream/master'
5f109b4 [Shuo Xiang] Merge remote-tracking branch 'upstream/master'
c5c5bfe [Shuo Xiang] Merge remote-tracking branch 'upstream/master'
98804c9 [Shuo Xiang] fix bug in topBykey and update test