docs: add versioning policy

docfx~/toc.yml

@@ -8,4 +8,6 @@
     - name: Execution Model
       href: execution-model.md
     - name: Extension Contexts
-      href: extension-context.md
+      href: extension-context.md
+    - name: Versioning Policy
+      href: versioning-policy.md

docfx~/versioning-policy.md

@@ -0,0 +1,21 @@
+## Versioning policy
+
+NDMF generally complies with [Semantic Versioning 2.0.0](https://semver.org/spec/v2.0.0.html). Versions are expressed as
+a MAJOR.MINOR.PATCH version triplet.
+
+* The major version will be incremented when non-backwards-compatible changes are made. For example, removing API
+  members, or adding abstract methods to abstract classes.
+* The minor version will be incremented when backwards-compatible changes are made. For example, adding new API members
+  to a class, or adding new abstract classes. It will also be incremented when changes to execution order heuristics
+  are made.
+* The patch version will be incremented when backwards-compatible bug fixes are made.
+
+Execution order heuristics are used to resolve the order of execution of passes when constraints are not sufficient to
+fully determine the order. For example, if two passes are declared to run after a third pass, but no other constraints
+are declared, the order of execution of the two passes is not fully determined. In this case, NDMF will use a heuristic
+to determine the order of execution. NDMF will strive not to change this heuristic between patch releases, but it may
+change on minor releases. If you run into problems with changes in a heuristic, please constrain the order of execution
+of your passes explicitly.
+
+**Versions before 1.0.0**: Versions starting with 0.x.y are considered to be unstable and do not guarantee API stability.
+We will make a best-effort attempt to increment the "x" version when making incompatible changes.