CONTRIBUTING.md: some tips on writing patches and workflow

This patch adds a new section in CONTRIBUTING.md about some principles on how
to write / arrange patches.

"Git Help" section has some overlapped content with the newly added section so
change it to "Basic Git Workflow". Add in some more commands in the example.

Also fixed some upper / lower case issues while I was there.

Signed-off-by: Wei Liu <liuw@liuw.name>