Update docs/README.md to put all prereqs together.

This pull request groups all the prereq requirements into a single section.

cc srowen shivaram

Author: Reynold Xin <[email protected]>

Closes apache#7951 from rxin/readme-docs and squashes the following commits:

ab7ded0 [Reynold Xin] Updated docs/README.md to put all prereqs together.

docs/README.md

@@ -9,12 +9,13 @@ documentation yourself. Why build it yourself? So that you have the docs that co
 whichever version of Spark you currently have checked out of revision control.
 
 ## Prerequisites
-The Spark documenation build uses a number of tools to build HTML docs and API docs in Scala, Python
-and R. To get started you can run the following commands
+The Spark documentation build uses a number of tools to build HTML docs and API docs in Scala,
+Python and R. To get started you can run the following commands
 
     $ sudo gem install jekyll
     $ sudo gem install jekyll-redirect-from
     $ sudo pip install Pygments
+    $ sudo pip install sphinx
     $ Rscript -e 'install.packages(c("knitr", "devtools"), repos="http://cran.stat.ucla.edu/")'
 
 
@@ -29,17 +30,12 @@ you have checked out or downloaded.
 In this directory you will find textfiles formatted using Markdown, with an ".md" suffix. You can
 read those text files directly if you want. Start with index.md.
 
-The markdown code can be compiled to HTML using the [Jekyll tool](http://jekyllrb.com).
-`Jekyll` and a few dependencies must be installed for this to work. We recommend
-installing via the Ruby Gem dependency manager. Since the exact HTML output
-varies between versions of Jekyll and its dependencies, we list specific versions here
-in some cases:
+Execute `jekyll build` from the `docs/` directory to compile the site. Compiling the site with
+Jekyll will create a directory called `_site` containing index.html as well as the rest of the
+compiled files.
 
-    $ sudo gem install jekyll
-    $ sudo gem install jekyll-redirect-from
-
-Execute `jekyll build` from the `docs/` directory to compile the site. Compiling the site with Jekyll will create a directory
-called `_site` containing index.html as well as the rest of the compiled files.
+    $ cd docs
+    $ jekyll build
 
 You can modify the default Jekyll build as follows:
 
@@ -50,29 +46,6 @@ You can modify the default Jekyll build as follows:
     # Build the site with extra features used on the live page
     $ PRODUCTION=1 jekyll build
 
-## Pygments
-
-We also use pygments (http://pygments.org) for syntax highlighting in documentation markdown pages,
-so you will also need to install that (it requires Python) by running `sudo pip install Pygments`.
-
-To mark a block of code in your markdown to be syntax highlighted by jekyll during the compile
-phase, use the following sytax:
-
-    {% highlight scala %}
-    // Your scala code goes here, you can replace scala with many other
-    // supported languages too.
-    {% endhighlight %}
-
-## Sphinx
-
-We use Sphinx to generate Python API docs, so you will need to install it by running
-`sudo pip install sphinx`.
-
-## knitr, devtools
-
-SparkR documentation is written using `roxygen2` and we use `knitr`, `devtools` to generate
-documentation. To install these packages you can run `install.packages(c("knitr", "devtools"))` from a
-R console.
 
 ## API Docs (Scaladoc, Sphinx, roxygen2)