The default branch was renamed from `master` to `main`.  Update some
stale references.

Signed-off-by: Sven van Haastregt <sven.vanhaastregt@arm.com>

The instructions on merging a PR are out of date. They have be updated
to match the current process.

Visual Studio 2013 is not supported anymore.

I'm about to change this file.  I re-formatted it for easier editing.

This is intended to be a document that people who want to help the project
can get a quick overview of how things are done.  Please comment and suggest
any changes.