Proposal - Maintaining a ChangeLog

I wrote an initial ChangeLog in #504 to help us follow through the tons of changes between versions.

I went through the closed PR since GitPoint was published on github, and I curated the list.
(@housseindjirdeh I'm not sure what was in 1.2 and what was in 1.3 though).

The idea would be to update this file every time a PR is merged carrying a _significant_ change to GitPoint. (and RN versions upgrades which seemed important to me)

These changes should be relevant to the end-user, and be formatted as follow:
- Added XXXXXX (#PR_NUMBER)
- Fixed XXXXXX (#PR_NUMBER)

Minor cosmetic/ui/ux fixes and changes shouldn't be written down. Neither should build related stuff.

The ChangeLog should also be updated with every release of the app to add the version number, release date & create a new master section.

Opinions ?

Start with the initial ChangeLog referenced in issue #504, then review the closed pull requests since GitPoint was published to compare the curated entries. Confirm how release versions and dates should be grouped, and document the agreed format for significant user-facing changes while excluding cosmetic and build changes.

Written by the indexing model from the issue text.