Write docs for transition to new crate structure

[thomaseizinger]

Created as a result of #272 (review).

This issue is to collect what needs to be done before we can cut the first release:

- [x] Document how users can replace the identity hasher. Done in https://github.com/multiformats/rust-multihash/pull/289.
- [ ] https://github.com/multiformats/rust-multihash/pull/320
- [ ] https://github.com/multiformats/rust-multihash/pull/321
- [ ] https://github.com/multiformats/rust-multihash/pull/319

[thomaseizinger]

README needs to be updated too.

[vmx]

Add a note that users upgrading could define a type alias like type Multihash = multihash::Multihash<64>. See
#305 (comment) for more information.

[thomaseizinger]

Where do you think would this migration docs best be written? In the README? Normally I'd add it to the changelog but that one is auto-generated here, right?

[vmx]

Normally I'd add it to the changelog but that one is auto-generated here, right?

The changelog. I'd call it "semi-autogenerated". So what I would do is, creating the new entries as outlined in the RELEASE.md and then manually tweak it. Please note that I've tried to make it clearer (and also updated the instructions for the generation) at #314.

[thomaseizinger]

@vmx all items in here now have an open PR. Do you mind having a look and see if that is sufficient documentation so we can cut a release?

[thomaseizinger]

Closing this because all tasks are completed.