Add CHANGELOG.md + Update version

2.1.0

Changed:
- Soft deprectated passing options with multiple arguments, prefer using named options
  now.
- Added configuration option for ETF encoding.
  You can now communicate with an outdated program by passing `etf_opts: [version: 1]`
  for example.

release 1.0.3

CHANGES:
	remove warning due to depreacteion of GenEvetn

bump version

v1.0.0

- A GenEvent can be attached to handle port messages sent outside a call
- Reference implementation of a port in Elixir is now provided in tests
- Exos Unit Testing added
- new documentation in Exos.Proc.start_link