Add notes for the change

notes/0.13.9/java-constant-fields.markdown

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+  [@stuhood]: https://github.com/stuhood
+  [@jsuereth]: https://github.com/jsuereth
+  [@adriaanm]: https://github.com/adriaanm
+
+  [1967]: https://github.com/sbt/sbt/issues/1967
+  [2085]: https://github.com/sbt/sbt/pull/2085
+
+### Changes with compatibility implications
+
+### Improvements
+
+### Fixes
+
+- Changing the value of a constant (final-static-primitive) field will now
+  correctly trigger incremental compilation for downstream classes. This is to
+  account for the fact that java compilers may inline constant fields in
+  downstream classes. [#1967][1967]/[#2085][2085] by [@stuhood][@stuhood]