Added versioning documentation [ci skip]

RELEASING.md

@@ -1,3 +1,28 @@
+# Versioning
+
+Pillow follows Semantic Versioning. From https://semver.org/:
+
+> Given a version number MAJOR.MINOR.PATCH, increment the:
+> 1. MAJOR version when you make incompatible API changes,
+> 2. MINOR version when you add functionality in a backwards compatible manner, and
+> 3. PATCH version when you make backwards compatible bug fixes.
+
+Quarterly releases (referred to as "Main Release" in the checklist below) bump at
+least the MINOR version, as new functionality has likely been added in the prior three
+months.
+
+A quarterly release bumps the MAJOR version when make incompatible API changes are
+made, such as removing deprecated APIs or dropping an EOL Python version. In practice,
+these occur every 12-18 months, guided by Python's EOL schedule, and any APIs that have
+been deprecated for at least a year are removed at the same time.
+
+PATCH versions ("Point Release" or "Embargoed Release" in the checklist below) are for
+security, installation or critical bug fixes. These are less common as it is preferred
+to stick to quarterly releases.
+
+Between quarterly releases, ".dev0" is appended to the version, indicating that this is
+not a formally released copy.
+
 # Release Checklist
 
 ## Main Release