Add making changes section to DeveloperGuide

[mertakman]

This part of document covers:

• Preparing go/src folder without falling much duplication with previous sections,
• Making changes on go/src and small information about our approach
• Generating new patch files and squashing changes to existing patch files
• Rolling back changes in go/src after generating patch files

[dagood]

I think the doc needs to get into git go-patch rebase, and I have some editing thoughts, but can save those for future PRs. Main comment is the one about cleanup.

[qmuntal]

Approving, although I think that the last section still needs some rewording (which can happen in a follow up PR).