Add branch to contributing guidelines

[tmorrell]

What branch should be used for contributions?

In the past the 'develop' branch was used as the target for pull requests, but recently it looks like the 'master' branch has been used. Once we decide, this should be documented in the contributing guidelines.

[progval]

Yeah I kind of started using master as the branch to land development because there isn't much point in having a branch just to serve released stuff (that's what tags are for).

We might want to keep develop for vocabulary changes, so that we can still release non-vocabulary changes (ie. mostly crosswalks) in minor releases without being stuck with stabilization of unrelated changes to the vocabulary

[bact]

Will these make sense?

Vocabulary changes:

• Add/amend/remove property
• Amend property name
• Amend property type
• Amend property's parent type
• Amend property description, in a way that will change the semantic of the property

Non-vocabulary changes is all other changes that is not included in the vocabulary changes list above, including:

• Small amendment to property description, that will NOT change the semantic of the property
• Crosswalk and mapping updates
• Documentation changes (README, etc)

[bact]

I have proposed PR #352 for this issue. Can be change upon the discussion here.

[moranegg]

Thanks @bact, I have approved the review. will be merged soon.

[bact]

Thanks @moranegg, I just spot one typo in the PR. Fixed.