Go: add change-note for path transformer fixes

Author: nickrolfe

go/ql/lib/change-notes/path-transformer.md

@@ -0,0 +1,7 @@
+---
+fix
+---
+* Some fixes relating to use of path transformers when extracting a database:
+  * Fixed a problem where the path transformer would be ignored when extracting older codebases that predate the use of Go modules.
+  * The environment variable `CODEQL_PATH_TRANSFORMER` is now recognized, in addition to `SEMMLE_PATH_TRANSFORMER`.
+  * Fixed some cases where the extractor emitted paths without applying the path transformer.