docs: Documented cat and credential template functions

docs/Templates.md

@@ -73,5 +73,33 @@ Additionally, the result is piped through the built-in Go template function `js`
 
 ```
 
+## Template Functions
+
+In addition to the [built-in Go template functions and features][tt], `webhook` provides the following functions:
+
+### `getenv`
+
+The `getenv` template function can be used for inserting environment variables into a templated configuration file.
+
+### `cat`
+
+The `cat` template function can be used to read a file from the local filesystem. This is useful for reading secrets from files. If the file doesn't exist, it returns an empty string.
+
+Example:
+```json
+"secret": "{{ cat "/run/secrets/my-secret" | js }}"
+```
+
+### `credential`
+
+The `credential` template function provides a way to retrieve secrets using [systemd's LoadCredential mechanism](https://www.freedesktop.org/software/systemd/man/systemd.exec.html#Credentials). It reads the file specified by the given name from the directory specified in the `CREDENTIALS_DIRECTORY` environment variable.
+
+If `CREDENTIALS_DIRECTORY` is not set, it will fall back to using `getenv` to read the secret from an environment variable of the given name.
+
+Example:
+```json
+"secret": "{{ credential "my-secret" | js }}"
+```
+
 [w]: https://github.com/adnanh/webhook
 [tt]: https://golang.org/pkg/text/template/