Wrote up versioning info

Versioning.md

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+# Versioning Style
+
+**footnotes** uses [Semantic Versioning 2.0.0](https://semver.org/).
+
+A version identifier consists of three integers, separated by dots:
+
+- The first integer represents a Major Version.
+- The second integer represents a Minor Version.
+- The third integer represents a Bugfix Version.
+
+On top of this, **footnotes** version identifiers can be appended with one of two flags:
+
+- `d`, indicating a development release
+    - e.g., ‘2.5.2d’
+    - This should **only** be found on the Git repo.
+    - The version number should **always** be one higher than the version number listed in the ‘Stable Tag’ field in `readme.txt`
+- `p`, indicating a pre-release
+    - e.g., ‘2.5.2p’
+    - This should only be found on the SVN repo.
+    - The version number should **always** be one higher than the version number listed in the ‘Stable Tag’ field in `readme.txt`
+
+So, to summarise:
+- if 2.5.1 is the latest stable release available on the SVN repo. (from `tags/2.5.1/`);
+- the codebase in the `main` branch on the Git repo. should be tagged as 3.0.0d, 2.6.0d or 2.5.2d; 
+- the codebase in `trunk/` on the SVN repo. should be 2.5.1 and identical to the release in `tags/2.5.1/`; OR
+- the codebase in `trunk/` should be tagged as 3.0.0p, 2.6.0p or 2.5.2p, with the pre-release copied to `tags/3.0.0p/`, `tags/2.6.0p/` or `tags/2.5.2p/`; and
+- the ‘Stable Tag’ field in `trunk/readme.txt` should continue to point to `tags/2.5.1`.
+
+When a pre-release version is confirmed to be stable, rename the folder in `tags/` to remove the `p` flag and update the ‘Stable Tag’ field in `trunk/readme.txt`.