Add a document describing the release process

Author: Philip de Nier

README.md

@@ -244,6 +244,8 @@ Source distributions, including dependencies, and binaries are made available in
 
 Source and binary distributions are generally only created when a new feature is required for creating standard compliant sample files for example, or when a release hasn't been made for a long time.
 
+The [release.md](./docs/release.md) document describes the release process.
+
 ### Conan
 
 Additionally bmx (including MXF and MXF++) is available via the Conan package manager for C/C++: [bmx Conan recipe](https://conan.io/center/recipes/bmx). Conan Center Index offers precompiled packages for many target systems and otherwise allows to build locally according to a local conan build profile. Follow the code snippets and documentation on the linked page for integrating it.

docs/release.md

@@ -0,0 +1,75 @@
+# Release Process
+
+This describes the steps for making a release.
+
+## Create a Release PR
+
+* Create a branch that will contain changes for the release
+* Update the [CHANGELOG.md](../CHANGELOG.md) using the most recent version as a guide
+    * Go through the PRs since the last release and add each PR and descriptive text to the `Breaking changes`, `Features`, `Bug fixes` or `Build changes` sections
+* Update the versions in the 3 main CMakeLists.txt in the `project` blocks, where `VERSION` has the form `<major version>.<minor version>`
+    * The CMakeLists.txt are as follows: [bmx CMakeLists.txt](../CMakeLists.txt), [libMXF CmakeLists.txt](../deps/libMXF/CMakeLists.txt) and [libMXFpp CmakeLists.txt](../deps/libMXFpp/CMakeLists.txt)
+* Run the [Build & Test](https://github.com/bbc/bmx/actions/workflows/build_and_test.yml) workflow in GitHub Actions using the release branch and fix any build errors and warnings
+* Check the [runner versions](https://docs.github.com/en/actions/using-github-hosted-runners/using-github-hosted-runners/about-github-hosted-runners) (e.g. `windows-2019` and `macos-13`) in the [release workflow](../.github/workflows/release.yml) are still available and update the workflow if not
+    * Select the oldest macOS version available to help compatibility with older macOS versions
+* Run the [Release](https://github.com/bbc/bmx/actions/workflows/release.yml) workflow in GitHub Actions using the release branch to check it succeeds
+* Create and merge a PR into `main` for the release branch
+
+## Create a Release Tag
+
+* Checkout and fetch the `main` branch
+* Create a tag with form `v<major version>.<minor version>`. E.g. export `BMX_VERSION` (without the `v`), replacing `<major version>.<minor version>` below
+
+```bash
+export BMX_VERSION=<major version>.<minor version>
+git checkout main
+git pull --rebase
+git tag -a v${BMX_VERSION} -m "Version ${BMX_VERSION}"`
+git push origin v${BMX_VERSION}
+```
+
+## Create the Release Packages
+
+* Run the [Release](https://github.com/bbc/bmx/actions/workflows/release.yml) workflow in GitHub Actions
+* Download the Artifacts and extract the individual source and binary zips for the release
+
+## Create a GitHub Release
+
+* Create a new release
+* Write the release text using the previous release's text as a starting point
+    * Select the `v<major version>.<minor version>` tag
+    * Change the CHANGELOG link
+    * Update the zip filenames with the new version
+    * Update the compiler versions used for the binaries
+        * These can be found in the actions output in the `Win64 binary release` and `MacOS Universal binary release` build steps in the 2 jobs of the [Release](https://github.com/bbc/bmx/actions/workflows/release.yml) workflow in GitHub Actions
+* Upload the source and binary zips to the release
+
+## Create a Docker Image for the GitHub Container Registry
+
+* Aquire a Linux machine with Docker installed
+* Build the `bmxtools` image and tag using the commands below
+    * `BMX_VERSION` should be set as before; note also the additional `.0` for the patch version.
+
+```bash
+DOCKER_BUILDKIT=1 docker build -t bmxtools .
+docker tag bmxtools ghcr.io/bbc/bmxtools:${BMX_VERSION}.0
+docker tag bmxtools ghcr.io/bbc/bmxtools:latest
+```
+
+* On your [GitHub settings](https://github.com/settings/tokens) to create tokens (or navigate from your personal page, go to "Settings" (top right) -> "Developer settings" (bottom) -> "Personal Access Tokens" / "Tokens (classic)")
+    * Select "Generate New Token" and "Generate new token (classic)"
+        * Change "Expiration" to 7 days
+        * Select "write:packages" (which will select "read:packages" and all of "repo")
+    * Select "Generate Token"
+    * Copy the token into your clipboard
+* Login into GHCR and push the image using the commands below
+    * Replace `<username>` with your GitHub username
+    * Pass in the token when docker login requests it
+    * `BMX_VERSION` should be set as before; note also the additional `.0` for the patch version
+
+```bash
+docker login ghcr.io --username <username>
+docker push ghcr.io/bbc/bmxtools:${BMX_VERSION}.0
+docker push ghcr.io/bbc/bmxtools:latest
+docker logout ghcr.io
+```