docs: Document Release Process (#131)

RELEASING.md

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+# Releasing
+
+This document describes the release process for [`pbjson`], [`pbjson-types`] and
+[`pbjson-build`] to crates.io.
+
+Note that `pbjson-test` is not released as it is only used for testing.
+
+[`pbjson`]: https://crates.io/crates/pbjson
+[`pbjson-types`]: https://crates.io/crates/pbjson-types
+[`pbjson-build`]: https://crates.io/crates/pbjson-build
+
+## Step 1: Update Version
+
+First make a PR to update the version (example [#127](https://github.com/influxdata/pbjson/issues/127))
+
+## Step 2: Release
+Run the following commands to release `pbjson`, `pbjson-types` and `pbjson-build`
+
+```shell
+cargo publish -p pbjson
+cargo publish -p pbjson-build
+cargo publish -p pbjson-types
+```