add a note about go upgrades

DEVELOPING.md

@@ -9,6 +9,12 @@ You should use the [latest distribution of the Go language for your OS and archi
 
 Oragono vendors all its dependencies. Because of this, Oragono is self-contained and you should not need to fetch any dependencies with `go get`. Doing so is not recommended, since it may fetch incompatible versions of the dependencies.
 
+If you're upgrading the Go version used by Oragono, there are several places where it's hard-coded and must be changed:
+
+1. `.travis.yml`, which controls the version that our CI test suite uses to build and test the code (e.g., for a PR)
+2. `distrib/docker/Dockerfile`, which controls the version that the Oragono binaries in our Docker images are built with
+3. `go.mod`: this should be updated automatically by Go when you do module-related operations
+
 
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