RFR: 7903693 Add contribution guidelines section [v2]

[Ludvig Janiuk]

> Adding in `building.md` as it seems the place one would read if making changes.

Ludvig Janiuk has updated the pull request incrementally with three additional commits since the last revision:

 - x
 - x
 - final touches

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

Changes:
  - all: https://git.openjdk.org/jtreg/pull/192/files
  - new: https://git.openjdk.org/jtreg/pull/192/files/4233dae7..f77ffb94

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=192&range=01
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=192&range=00-01

  Stats: 8 lines in 1 file changed: 0 ins; 2 del; 6 mod
  Patch: https://git.openjdk.org/jtreg/pull/192.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/192/head:pull/192

PR: https://git.openjdk.org/jtreg/pull/192