docs: updated README

Author: slisson

README.md

@@ -1,2 +1,14 @@
 # modelix.mps-api
 Single jar library that works in a wide range of MPS versions
+
+In Modelix we switched from the strategy of maintaining a separate branch for each MPS version
+to a single release of our plugins that is compatible to a wide range of MPS versions.
+
+This works as long as we only use APIs that didn't change between MPS versions.
+Often it's possible to avoid unstable APIs,
+but sometimes there is a single API that can't be avoided and would make the plugin incompatible with previous versions.
+
+This library maintains these differences between MPS versions and provides a common API.
+For each MPS version there is a different implementation and all of them are packaged into the same JAR.
+At runtime the correct implementation is chosen based on the version number provided by 
+`com.intellij.openapi.application.ApplicationInfo`.