Change versioning scheme description in the README (#199)

* Change versioning scheme description in the README

* Moving old versioning scheme to the bottom of the README

* Add note on why we dropped the third digit in the preCICE version

README.md

@@ -4,7 +4,7 @@
 
 [![Upload Python Package](https://github.com/precice/python-bindings/workflows/Upload%20Python%20Package/badge.svg?branch=master)](https://pypi.org/project/pyprecice/)
 
-This package provides python language bindings for the C++ library [preCICE](https://github.com/precice/precice). Note that the first three digits of the version number of the bindings indicate the preCICE version that the bindings support. The last digit represents the version of the bindings. Example: `v2.0.0.1` and `v2.0.0.2` of the bindings represent versions `1` and `2` of the bindings that are compatible with preCICE `v2.0.0`.
+This package provides python language bindings for the C++ library [preCICE](https://github.com/precice/precice). Note that the first two digits of the version number of the bindings indicate the major and minor version of the preCICE version that the bindings support. The last digit represents the version of the bindings. Example: `v3.1.0` and `v3.1.1` of the bindings represent versions `0` and `1` of the bindings that are compatible with preCICE `v3.1.x`. Note that this versioning scheme was introduced from bindings `v3.1.0`, which is different than the [old versioning scheme](#old-versioning-scheme).
 
 ## User documentation
 
@@ -319,6 +319,10 @@ make sure that you are using an up-to-date version of NumPy. You can update NumP
 pip3 install numpy --upgrade
 ```
 
+## Old versioning scheme
+
+Bindings versions up to `v3.0.0.0` have four digits, where the first three digits are the supported preCICE version, and the fourth digit is the bindings version. Example: `v2.0.0.1` and `v2.0.0.2` of the bindings represent versions `1` and `2` of the bindings that are compatible with preCICE `v2.0.0`. We dropped the third digit of the preCICE version as bugfix releases are always compatible and do not impact the bindings. The new three digit format is now consistent with other preCICE bindings.
+
 ## Contributors
 
 * [Benjamin Rodenberg](https://github.com/BenjaminRodenberg)