Add pull request guidelines.

Add pull request guidelines. This documents guidelines that are being
proposed for pull requests. It also adds a "Process" directory for
housing guidelines that don't fit with a specific language or
technology.
Update Pull-Request.md to fix the indentation

Fix the list indentation.
Make the /cc @ message optional

Teams have different ways of handling this. @ messaging is one way to do
it but shouldn't be required.

/cc @garthwebb
Remove double "of" & move @ message example

Remove double "of" and move the @ message example up to 3.1.

/cc @grunny
Merge pull request #45 from Wikia/pull-request-guidelines

Add pull request guidelines & update the top level navigation