added guidelines for developers

- moved instructions from README.md to CONTRIBUTING.md
- added some further info on commit style and branching with git
Modified Hitmaker to work correctly with the information in cellid field
Removed clusterID from clustering processor
Removed clusterID and fixed interfacing the cellID field for SparsePixelType, detectorID and Quality field
Removed clusterID
Removed ClusterID from APIX tb dumper
Copying CellID fields in EUTelApplyAlignmentProcessor, fixes issue #38
Updated EUTelMille to use cellID instead of guessSensorID, issue #25
Removed obsolete lines
Replaced guessSensorID with cellID in ApplyAlignmentProcessor, verification test still included, has to be removed #25
Replaced guessSensorID in correlator #25
Removed guessSensorID from src/EUTelAPIXTbTrackTuple.cc #25
Removed guessSensorID from src/EUTelPreAlignment.cc #25
Removed guessSensorID from src/EUTelProcessorApplyAlignment.cc #25
Copying cellID fields to output collection in EUTelProcessorApplyAlignment.cc (similar to #38)
Merged cellID to guessSensor branch
Merged new guessSensor changes into dev branch