Adds doc with suggestions for stdlib dev.

doc/guide/hacking-on-the-stdlib.md

@@ -0,0 +1,33 @@
+# Hacking on the Standard Library
+
+Gerbil is a growing project and your contributions are welcome!
+
+When working on modifications to the standard library, it's common to encountere errors
+such as:
+
+```
+*** ERROR IN gx#core-expand-import%__% -- 
+*** ERROR IN "file.ss"@1.9
+--- Syntax Error: Cannot find library module
+... form:   :std/my/module
+```
+This typically because the `gxi` or `gxc` being used is looking at the installed
+standard library which will not contain your changes. To address this, run your
+environment with the `build.sh` script after building.
+
+Example build:
+
+```shell
+./build.sh
+```
+
+To test your local changes rebuild the stdlib and then use the locally built `gxi` with
+the following commands:
+
+```shell
+./build.sh stdlib
+./build.sh env gxi ../your-test-file.ss
+```
+**NOTE:** the execution environment when using `./build.sh env` is in the `src/` folder
+in the root of the Gerbil repo. Absolute paths work as normal, but relative paths must
+be adjusted for `PWD` change.