Explain version schema (#2284)

Author: janiversen

README.rst

@@ -11,6 +11,18 @@ PyModbus - A Python Modbus Stack
 
 Pymodbus is a full Modbus protocol implementation offering client/server with synchronous/asynchronous API a well as simulators.
 
+Our releases is defined as X.Y.Z, and we have strict rules what to release when:
+
+- **Z**, No API changes! bug fixes and smaller enhancements.
+- **Y**, API changes, bug fixes and bigger enhancements.
+- **X**, Major changes in API and/or method to use pymodbus
+
+Upgrade examples:
+
+- 3.6.1 -> 3.6.9: just plugin the new version, no changes needed.
+- 3.6.1 -> 3.7.0: Smaller changes to the pymodbus calls might be needed
+- 2.5.4 -> 3.0.0: Major changes in the application might be needed
+
 Current release is `3.7.0 <https://github.com/pymodbus-dev/pymodbus/releases/tag/v3.7.0>`_.
 
 Bleeding edge (not released) is `dev <https://github.com/pymodbus-dev/pymodbus/tree/dev>`_.