added guidelines for developers

- moved instructions from README.md to CONTRIBUTING.md
- added some further info on commit style and branching with git
Revert "hitmaker now adds sensorID and properties fields"

This reverts commit 6e9e440015fa4f50743f9149626c389e5759f26d.
This was a premature accidental commit. Sorry.
Revert "changed default cellID encoding and deconding strings, added hit properties"

This reverts commit cfc3e6d3b26863a505ba8a3d36cb8e324fd52f3f.
This was a premature and accidental commit. Sorry.
doc: cosmetic modifications to guidelines

- added line breaks after links