"Contributing to OMERO" developer doc rework (rebased onto develop)



This is the same as gh-222 but rebased onto develop.

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 * Rename files to follow abc-xyz.txt standard
 * Reduce the number of individual files
 * Improve the order of files for external developers
 * Improve the order of sections in some files
 * Drop UsingGitFlow

Looking at developers/index.txt and developers/contributing.txt
it's still not completely clear 1) that a developer should REALLY
read the "contributing" section, nor 2) once they've opened up
contributing.txt which sections are highly important.

NB: very little has been done in the way of optimizing the PDF