Updates README.md with instructions for running jekyll without building

scaladoc (i.e. run `SKIP_SCALADOC=1 jekyll`).

docs/README.md

@@ -10,7 +10,7 @@ We include the Spark documentation as part of the source (as opposed to using a
 
 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.
 
-To make things quite a bit prettier and make the links easier to follow, generate the html version of the documentation based on the src directory by running `jekyll` in the docs directory. To do so, you will need to have Jekyll installed, the easiest way to do this is via a Ruby Gem, see the [jekyll installation instructions](https://github.com/mojombo/jekyll/wiki/install). This will create a directory called _site containing index.html as well as the rest of the compiled files. Read more about Jekyll at https://github.com/mojombo/jekyll/wiki.
+To make things quite a bit prettier and make the links easier to follow, generate the html version of the documentation based on the src directory by running `jekyll` in the docs directory. Use the command `SKIP_SCALADOC=1 jekyll` to skip building and copying over the scaladoc which can be timely. To use the `jekyll` command, you will need to have Jekyll installed, the easiest way to do this is via a Ruby Gem, see the [jekyll installation instructions](https://github.com/mojombo/jekyll/wiki/install). This will create a directory called _site containing index.html as well as the rest of the compiled files. Read more about Jekyll at https://github.com/mojombo/jekyll/wiki.
 
 In addition to generating the site as html from the markdown files, jekyll can serve up the site via a webserver. To build and run a webserver use the command `jekyll --server` which (currently) runs the webserver on port 4000, then visit the site at http://localhost:4000.
 
@@ -30,3 +30,5 @@ To mark a block of code in your markdown to be syntax highlighted by jekyll duri
 You can build just the Spark scaladoc by running `sbt/sbt doc` from the SPARK_PROJECT_ROOT directory.
 
 When you run `jekyll` in the docs directory, it will also copy over the scala doc for the various Spark subprojects into the docs directory (and then also into the _site directory). We use a jekyll plugin to run `sbt/sbt doc` before building the site so if you haven't run it (recently) it may take some time as it generates all of the scaladoc.
+
+NOTE: To skip the step of building and copying over the scaladoc when you build the docs, run `SKIP_SCALADOC=1 jekyll`.