Update README.md

Projectile's `C-c A` by default is bound to `ag-regexp` as you can see here https://github.com/bbatsov/projectile/blob/b1fb903530def09d815b359524d16edb099f5322/projectile.el#L1075
Merge pull request #42 from tomjakubowski/patch-1

Update README.md
Clarify that regular expressions must be PCRE syntax. #40
Allow the user to override any ag argument when called with a prefix.

Fixes #38.

This saves the user having to edit a list, and allows them to see the
whole command before executing it (so any part may be changed).
Version bump and changelog update.