[yocto] [meta-raspberrypi] Pull requests and jenkins

[Andrei Gherzan]

Hi all,

I know I promised long ago but finally I took a Friday to invest into
github pull requests integrated with a jenkins server.

The first PR on which I worked for this integration is
https://github.com/agherzan/meta-raspberrypi/pull/67 . This PR:
- migrates Readme to markdown file - any objections about this?
- adds badges for jenkins builds and gitter room
- add info about pull requests

As well I pushed a PR from an external fork for testing -
https://github.com/agherzan/meta-raspberrypi/pull/68 . This adds a log
entry in gitignore which won't hurt anyway.

Now, what I propose  is to start using pull requests as main way of
contributing and mailing list as an additional (optional) way of
advertising changes. We will keep it though for general discussions
but keep contributions and issues (bug tracking) in github.

About the jenkins server. The current setup is the following: for each
PR, jenkins triggers rpi-basic-image for rpi, rpi2 and rpi3 using the
target branch as the one on which the dependencies will be checked out
(poky, meta-oe etc). If all three builds succeed then the PR is
approved. I plan to start to add weekly builds for master and last two
branches (morty and krogoth at this point) and additional boards. But
for now I think it's a good start.

Thoughts?

--
Andrei Gherzan