Remove outdated information from README

[artob]

The current README file contains manifestly out-of-date information, such as:

• "We do not merge pull requests directly on Github, all PRs will be pulled and pushed through https://git.apache.org/." [No longer true, looking at the commit history.]
• "Bugs can be filed on the Apache Mynewt Bug Tracker." [No longer true, there's nothing much in the JIRA instance, versus many issues here at GitHub.]
• "Feature requests should also be filed on the Apache Mynewt Bug Tracker." [As the previous.]

[ccollins476ad]

Thanks @artob. I have submitted a PR to fix the issues you identified: #332

Better late than never!