[SPARK-7555] [DOCS] Add doc for elastic net in ml-guide and mllib-guide

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

1 files changed, 31 insertions, 0 deletions

diff --git a/docs/ml-guide.md b/docs/ml-guide.md
index c74cb1f1ef..8c46adf256 100644
--- a/docs/ml-guide.md
+++ b/docs/ml-guide.md
@@ -3,6 +3,24 @@ layout: global
 title: Spark ML Programming Guide
 ---
 
+`\[
+\newcommand{\R}{\mathbb{R}}
+\newcommand{\E}{\mathbb{E}}
+\newcommand{\x}{\mathbf{x}}
+\newcommand{\y}{\mathbf{y}}
+\newcommand{\wv}{\mathbf{w}}
+\newcommand{\av}{\mathbf{\alpha}}
+\newcommand{\bv}{\mathbf{b}}
+\newcommand{\N}{\mathbb{N}}
+\newcommand{\id}{\mathbf{I}}
+\newcommand{\ind}{\mathbf{1}}
+\newcommand{\0}{\mathbf{0}}
+\newcommand{\unit}{\mathbf{e}}
+\newcommand{\one}{\mathbf{1}}
+\newcommand{\zero}{\mathbf{0}}
+\]`
+
+
 Spark 1.2 introduced a new package called `spark.ml`, which aims to provide a uniform set of
 high-level APIs that help users create and tune practical machine learning pipelines.
 
@@ -154,6 +172,19 @@ Parameters belong to specific instances of `Estimator`s and `Transformer`s.
 For example, if we have two `LogisticRegression` instances `lr1` and `lr2`, then we can build a `ParamMap` with both `maxIter` parameters specified: `ParamMap(lr1.maxIter -> 10, lr2.maxIter -> 20)`.
 This is useful if there are two algorithms with the `maxIter` parameter in a `Pipeline`.
 
+# Algorithm Guides
+
+There are now several algorithms in the Pipelines API which are not in the lower-level MLlib API, so we link to documentation for them here.  These algorithms are mostly feature transformers, which fit naturally into the `Transformer` abstraction in Pipelines, and ensembles, which fit naturally into the `Estimator` abstraction in the Pipelines.
+
+**Pipelines API Algorithm Guides**
+
+* [Feature Extraction, Transformation, and Selection](ml-features.html)
+* [Ensembles](ml-ensembles.html)
+
+**Algorithms in `spark.ml`**
+
+* [Linear methods with elastic net regularization](ml-linear-methods.html)
+
 # Code Examples
 
 This section gives code examples illustrating the functionality discussed above.