Add note about Prometheus as a library (#10594)

* Add note about Prometheus as a library

Signed-off-by: Julien Pivotto <roidelapluie@o11y.eu>

README.md

@@ -122,6 +122,37 @@ You can build a docker image locally with the following commands:
 
 *NB* if you are on a Mac, you will need [gnu-tar](https://formulae.brew.sh/formula/gnu-tar).
 
+## Using Prometheus as a Go Library
+
+### Remote Write
+
+We are publishing our Remote Write protobuf independently at
+[buf.build](https://buf.build/prometheus/prometheus/assets).
+
+You can use that as a library:
+
+```shell
+$ go get go.buf.build/protocolbuffers/go/prometheus/prometheus
+```
+
+This is experimental.
+
+### Prometheus code base
+
+In order to comply with [go mod](https://go.dev/ref/mod#versions) rules,
+Prometheus release number do not exactly match Go module releases. For the
+Prometheus v2.y.z releases, we are publishing equivalent v0.y.z tags.
+
+Therefore, a user that would want to use Prometheus v2.35.0 as a library could do:
+
+```
+$ go get github.com/prometheus/prometheus@v0.35.0
+```
+
+This solution makes it clear that we might break our internal Go APIs between
+minor user-facing releases, as [breaking changes are allowed in major version
+zero](https://semver.org/#spec-item-4).
+
 ## React UI Development
 
 For more information on building, running, and developing on the new React-based UI, see the React app's [README.md](web/ui/README.md).