Proposed documentation updates

[RitaLav]

1. Add clarification in README about live abbreviations csv file vs abbreviations csv file associated with a specific release.
2. In README, add link to section 'Submission process for building device names' within 'BDNS_Governance_model' to create a quick reference to the process of adding an abbreviation to the register.
3. Consider updating 'Submission process for building device names' within 'BDNS_Governance_model' to reflect up to date steps (minor modification needed)
4. Add clarification to 'Versioning of documents' section within 'BDNS_Governance_model' in definitions of Editors draft and Final spec.