Updating contribution doc to include info on syncing R with GitHub branches

[Rainiefantasy]

Updating contribution doc to include info on syncing R with GitHub branches. This will make testing easier, and will make sure the branch is synced and package is the relevant version requiring testing.

The following change needs to be implemented:

• adding section in contribution doc to make testing on branches easier
• to include this particular line: devtools::load_all() which reloads/syncs the package in R studio or devtools::install_github("aim-rsf/browseMetadata", ref="branch-name") seems to have the same effect (installs and syncs with specified branch)

[RayStick]

Thanks @Rainiefantasy =D Let's add @BatoolMM as reviewer for this PR