Added a README in protoregistry folder

protoregistry/README.md

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+### Protobuf Registry
+
+The well-known Protobuf definitions are pulled from Buf Build Registry. This makes `firehose-core` able to decode some well-known block files directly.
+
+#### Re-generate
+
+To re-generate the well-known types, simply do:
+
+```bash
+go generate ./protoregistry
+```
+
+While being at the root of the **project** (if you run from this directory here, adjust `./protoregistry` to `.`). Before re-generating, ensure you have push to Buf Registry the latest version of the definitions.
+
+#### Add new well-known types
+
+Push your definitions to the Buf Registry. Edit file [./generator/generator.go](./generator/generator.go) and add the Buf Registry path of the package in `wellKnownProtoRepos` variables.
+
+Then [re-generate Protobuf definitions](#re-generate) and send a PR with the changes.