Adds local build instructions to README documentation

Change-Id: I907e9fd8cc47354ba15c2bca6f0ffb851820f22d
Reviewed-on: http://gerrit.causes.com/45080
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Copy adjustment for contributing section

Bring this in line with the style used throughout the documentation.

Change-Id: I9cb12507b0d294532e474f822612a21188217b32
Reviewed-on: http://gerrit.causes.com/45081
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>