request: simple contribution guide

[jeffbean]

Without a Make environment or a simple setup guide its a little up in the air if my failing tests are my environment or what without submitting a PR for the full build.

A simple setup guide or a Makefile for testing would be immensely helpful. Even if its "run travis target locally" here's how :D

[fujita]

slightly outdated but here's how to run the CIs locally
https://github.com/osrg/gobgp/blob/master/test/scenario_test/README.md
I guess that it would be better to have sorta developer guide on the top of the directory.