Merge pull request #7382 from sampablokuper/spk-note-version-numbering-system

Document the versioning system used by Composer.

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

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+# 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).