dictionary_development_practices.md: establish rules for changing the version number

[vaitkus]

This issue is based on the comments in issue COMCIFS/cif_core#312. The original message reads:

We need to improve the explanation of how to determine whether or not a change is major/minor/patch in the dictionary development guidelines, something like:

1. major: incompatible change in data name meaning
2. minor: addition of one or more new data names (except those already implied by existing names, i.e. _su)
3. version: improvement in existing data names, fixing of bugs

We are unlikely to ever change the major number as far as I can tell.

Similar rules are already provided in section Versioning a development dictionary, however, these may need expanding or rewriting.

[jamesrhester]

Our current development process consists of incrementing the minor number and then accumulating many updates under that minor number. This seems reasonable and keeps our minor number at a reasonable size. Incrementing the patch number would then only happen where there was a bad bug and we needed an immediate release to fix it.