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 ?