Update readme to warn against using different escript and deps versions.

[m1dnight]

Ive spent a good amount of time figuring out why our protobufs didnt compile anymore.

It turned out that the version being installed in the CI was 0.14 and not 0.11 as in the application.

I added a warning for this to the readme, because I feel it could save other people a lot of time.

Perhaps for the Google machine, the error I got was the following:

** (KeyError) key :features not found
    (protobuf 0.11.0) expanding struct: Google.Protobuf.MethodOptions.__struct__/1
    lib/protobuf/indexer_service.pb.ex:13: IndexerService.Service.descriptor/0```

[v0idpwn]

This seems about right.

I wonder if we could verify it. Generally we are backwards compatible in the library, but potentially not in the escript. So we'd need to check when compiling if the generated-module version is higher than the running library version, and warn.

[v0idpwn]

Thanks! 💜