Document the versioning system used by Composer.

Fixes #7356.

doc/faqs/which-version-numbering-system-does-composer-itself-use.md

@@ -0,0 +1,4 @@
+# Which version numbering system does Composer itself use?
+
+Composer uses [Semantic Versioning (aka SemVer)
+2.0.0](https://semver.org/spec/v2.0.0.html).