Add RELEASING.md documentation.

RELEASING.md

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+# Releasing OpenTelemetry Rust
+
+The primary audience for this is the SIG Maintaners. It provides the list of steps for how to release the crates and the
+considerations to make before releasing the create. It may provide use to consumers of the crate if/when we develop a
+release cadence.
+
+## Release cadence
+
+As of Feb 2024, there is no established cadence for the OpenTelemetry crates. The balance is required between too many
+breaking changes in a single release, and since we have instability flipping between implementations across 0.x
+releases. Perhaps once we've established a 1.0 we can consider a consistent release cadence.
+
+## Considerations
+
+A draft PR can be created, but before releasing consider the following:
+
+* Are there any pending pull requests which should be included in the next release?
+  * Are they blockers?
+* Are there any unresolved issues which should be resolved before the next release?
+* Bring it up at a SIG meeting, this can usually get some of these questions answered sooner than later. It will also
+  help establish a person to perform the release. Ideally this can be someone different each time to ensure that the
+  process is documented.
+
+## Steps to Release
+
+1. Create a release PR
+  - For each crate
+    - Bump appropriate version
+    - Update change logs to reflects
+    - Update API/SDK version as necessary
+  - If there's a large enough set of changes consider writing a migration guide.
+2. Merge the PR
+  - Get reviews from other Maintainers
+  - Ensure that there haven't been any interfering PRs
+3. Tag the release commit based on the [tagging convention](#tagging-convention)
+4. Create Github Release
+5. [Publish](#publishing-crates) to crates.io using the version as of the release commit
+6. Post to [#otel-rust](https://cloud-native.slack.com/archives/C03GDP0H023) on CNCF Slack.
+
+
+## Tagging Convention
+
+For each crate: it should be `<crate-name>-<version>` `<version>` being the simple `X.Y.Z`.
+
+## Publishing Crates
+
+For now we use the [basic procedure](https://doc.rust-lang.org/cargo/reference/publishing.html) from crates.io.
+
+Follow this for each crate as necessary.
+
+For any new crates remember to add open-telemetry/rust-maintainers to the list.