Add HACKING.md file

HACKING.md file contains the build instructions to build and run Geoclue

Fixes #104
4 jobs for add_HACKING.md_file in 6 minutes and 59 seconds (queued for 71 minutes and 31 seconds)
Status Job ID Name Coverage
  Test
passed #168069
ubuntu-16.04

00:01:21

passed #168070
ubuntu-17.10

00:01:36

passed #168071
ubuntu-18.04

00:02:06

passed #168072
ubuntu-18.04-no-backend

00:01:54