Clarify compatibility between protoc_plugin and protobuf versions

READMEs of [protobuf][1] and [protoc_compiler][2] do not say anything about the compatibility of these packages, i.e. for a version of one of those packages which version(s) of the other package I should be able to use.

It's also not helping that

- These libraries do not have the same version numbers. Current protoc_plugin is 20.0.1, protobuf is 2.0.1. Some of the previous versions of protoc_plugin are 19.3.1, 19.0.1 etc. and there are no similar looking version number in protobuf. We have 1.1.1, 0.14.4, 0.13.16 etc. It's impossible to tell what is compatible with what.

- 1-1 mapping is not possible. For example, protobuf-1.1.3 was released 13 months ago, but no protoc_plugin release was made at that time.

As a user, when the latest versions of protobuf and protoc_plugin work for me this is not a problem. But if I want to bump version numbers (e.g. newer versions have a bug fix) but not to the latest version (e.g. latest version is backwards incompatible with the version I'm using) then I'm in trouble as for the backwards-compatible version with the bug fix of one of these libraries I don't know which version of the other library I should be using.

[1]: https://github.com/google/protobuf.dart/blob/ded1ac7d2e00b1f8ff7e9ac4129a3be6c652ee94/protobuf/README.md
[2]: https://github.com/google/protobuf.dart/blob/ded1ac7d2e00b1f8ff7e9ac4129a3be6c652ee94/protoc_plugin/README.md