RFR: 6835: Adding developer guide for getting started with development

[Marcus Hirt]

This PR adds a short developer guide, to help newcomers getting started setting up the development environment.

-------------

Commit messages:
 - Adding some line breaks
 - 6835: link from project readme
 - 6835: Devguide - first draft

Changes: https://git.openjdk.java.net/jmc/pull/82/files
 Webrev: https://webrevs.openjdk.java.net/jmc/82/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6835
  Stats: 100 lines in 14 files changed: 84 ins; 14 del; 2 mod
  Patch: https://git.openjdk.java.net/jmc/pull/82.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/82/head:pull/82

PR: https://git.openjdk.java.net/jmc/pull/82