doc about version numbering

docs/internals/versions.md

+Yii version numbering
+=====================
+
+A.B.C
+
+A = For Yii2 it's always 2.
+B = Major version. Non-BC changes with upgrade instructions.
+C = BC changes and additions.
+
+A.B.CrcD
+
+This is when we want to release release candidate. D is the RC number. Starts with 1 and increments till we're getting a stable release with no critical bugs and BC incompatibility reports.
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