Update migration guide with the latest spec

[elsony]

The migration guide (https://devfile.github.io/devfile/migration_guide.html) was created a while ago. The guide needs to be updated to reflect the latest spec.

[yhontyk]

Hi @elsony! I'd like to pick this one up.

I've looked through the 2.0 milestone issues on GH.
The following changes do not seem to be mentioned in the Migration Guide:

1. Find a better naming for chePlugin spec reference
2. Rename command group type
3. configuration option to avoid unncessary restarts of process
4. default values on JSON schema
5. mountsources default value for containers
6. Manage merge keys of top-level lists in a more consistent way
7. Support multiple remotes for projects to support forking workflow
8. Exec command should make component required

Could you help me to identify those that need to go into the guide? I'll distil the descriptions from the issues/PRs, just need some help to figure out what's relevant from the docs perspective.

Also, are there any other places other than the epic and the milestone in GH where I can track the changes?

[rkratky]

Fixed by devfile/docs#22