README: Add dev setup/tips section + dev ideas section

[joshtrichards]

Some possible things to incorporate that others might find helpful:

• Development setup/tips
  • Development environment
    • Testing master
    • Testing stable branches
    • Testing your own changes
  • Server components of BFP
  • Organizing changes (branches, rebasing, commit messages, DCO signatures, and backports)
  • Tips for testing BFP changes / functionality
  • Submitting PRs
• Development ideas - e.g.
  • Unblock an IP address from admin settings page #9 (A means of resetting recent attempts history for an IP address from the Web UI)
  • View status of an IP address from web UI #560
  • essentially some highlighted ongoing areas of interest in seeing development progress in + a few choice highlights from the current "1. to develop" list or similar
  • Some more control and visibility #4
  • Any open bug
  • Attempting to reproduce any reported bug and either identifying the root cause or following up with the original reporter if more information is likely necessary to identify root cause