…ses and to be more compliant with ASGI spec

* Bugfix lifespan startup behaviour, allow lifespan cycle to be used as a context manager in the adapter [107](https://github.com/erm/mangum/issues/107).

* Deprecate `enable_lifespan` parameter to be replaced by new `lifespan` option

* Include CHANGELOG in repo and release notes in documentation [110](https://github.com/erm/mangum/issues/110)

* Update protocol classes generally with  docstrings/comments/better state transitions/more compliant with ASGI spec.

* Overhaul documentation structure and content.