Add Parallel Bundle Install as ruby til.

ruby/parallel-bundle-install.md

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+# Parallel Bundle Install
+
+The `bundle install` command can take quite a while sometimes. This is
+especially true for when you are setting up a fresh dev environment for an
+existing project. Fortunately, Bundler provides a flag for parallelizing
+the gem installs and Bundler is smart enough that it knows how to resolve
+the dependencies so that this is possible in the first place. Just tell
+Bundler how many concurrent jobs you want it to install with (4 is a good
+number) and let it do the rest of the work.
+
+```bash
+$ bundle install --jobs 4
+```