docs: initial release of CONTRIBUTING guide

* Adds `CONTRIBUTING.rst` guide, taken from our old Trac wiki site,
  reusing the old content, updating it only slightly so far.
  (closes #2163)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
Merge branch 'maint-1.0' into maint-1.1

* maint-1.0:
  docs: initial release of CONTRIBUTING guide

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
Merge branch 'maint-1.1'

* maint-1.1:
  docs: initial release of CONTRIBUTING guide

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>