Mention environment variables

Author: rfay

README.md

@@ -26,7 +26,7 @@ A repository like this one is the way to get started. You can create a new repo
 5. Update the install.yaml to give the necessary instructions for installing the add-on.
   * The fundamental line is the `files` directive, a list of files to be copied from this repo into the project `.ddev` directory.
   * You can optionally add files to the `global_files` directive as well, which will cause files to be placed in the global `.ddev` directory, `~/.ddev`.
-  * Finally, `pre_install_commands` and `post_install_commands` are supported.
+  * Finally, `pre_install_commands` and `post_install_commands` are supported. These can use the host-side environment variables documented [in ddev docs](https://ddev.readthedocs.io/en/stable/users/extend/custom-commands/#environment-variables-provided).
 6. Update `tests/test.bats` to provide a reasonable test for the repository. You can run it manually with `bats tests` and it will be run on push and nightly as well. Please make sure to attend to test failures when they happen. Others will be depending on you. `bats` is a simple testing framework that just uses `bash`. You can install it with `brew install bats-core` or [see other techniques](https://bats-core.readthedocs.io/en/stable/installation.html). See [bats tutorial](https://bats-core.readthedocs.io/en/stable/).
 7. When everything is working, including the tests, you can push the repository to GitHub.
 8. Create a release on GitHub.