Update README.md

Add info about volatile base units in the serialization section.

Author: angularsen

README.md

@@ -123,6 +123,8 @@ Serialization
 **Important!** 
 We cannot guarantee backwards compatibility, although we will strive to do that on a "best effort" basis and bumping the major nuget version when a change is necessary.
 
+The base unit of any unit should be be treated as volatile as we have changed this several times in the history of this library already. Either to reduce precision errors of common units or to simplify code generation. An example is Mass, where the base unit was first Kilogram as this is the SI unit of mass, but in order to use powershell scripts to generate milligrams, nanograms etc. it was easier to choose Gram as the base unit of Mass.
+
 What It Is Not
 ===
 * It is not an equation solver