Update BIP-0001 for github move

- Mention github instead of wiki
- Add markdown as allowed format
- Fix some weirdnesses such as "A BIP editor must subscribe to the
  gmaxwell@gmail.com list.". Remove the repetition and create a new
  section which lists the BIP editors (currently only gmaxwell).
- Prefer discussion on mailing list to forum
Merge pull request #1 from laanwj/master

Update BIP-0001 for github move