Explain why version tag is necessary

README.md

     $ git commit -m "Initial commit"
     $ git tag v0.0.0
 
+Note that a valid version tag like *v0.0.0* is necessary to help `vbump_*`
+targets compose template change overview message. (Future versions of MKit
+should probably learn to improvise a bit and get rid of this requirement
+though.)
+
 
 ### 2. embed MKit into your repo ###