Add HACKING.md file

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

Fixes #104
4 jobs for master in 5 minutes and 32 seconds (queued for 2 minutes and 56 seconds)
Status Job ID Name Coverage
  Test
passed #168178
ubuntu-16.04

00:01:24

passed #168179
ubuntu-17.10

00:01:39

passed #168180
ubuntu-18.04

00:01:16

passed #168181
ubuntu-18.04-no-backend

00:01:11