List requirements of building libSavitar

I just tried on a computer that had SIP 4.19.0 and that wasn't sufficient. I normally build with 4.19.2 and that works, and just now I upgraded that first computer to 4.19.3 so that works too. I don't know about 4.19.1 but 4.19.2 works in any case.

README.md

@@ -3,6 +3,11 @@ Introduction
 
 libSavitar is a c++ implementation of 3mf loading with SIP python bindings.
 
+Requirements
+------------
+
+libSavitar requires SIP version 4.19.2 or higher: https://www.riverbankcomputing.com/software/sip/download
+
 Build instructions Windows with MSVC
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