AP_Declination: added generator doc

libraries/AP_Declination/generate/README.md

@@ -0,0 +1,13 @@
+# Declination Table Generator
+
+To run the generator you need the igrf12 module. Install like this:
+
+  python3 -m pip install --user igrf12
+
+Note that python3 must be used.
+
+Then cd to libraries/AP_Declination directory, and run:
+
+ python3 generate/generate.py
+
+it will updates the tables.cpp code