docs: {pre, post}Hook under Usage section (#64)

* docs: `{pre, post}Hook` under `Usage` section

* sequence

---------

Co-authored-by: Sridhar Ratnakumar <[email protected]>

doc/index.md

@@ -74,6 +74,32 @@ devShells = {
 
 You can enter your devShell by running `nix develop` and run `watch-server` to run your processes.
 
+### preHook
+
+If you'd like to run certain commands before starting the processes, you can add them to `preHook`:
+
+```nix
+process-compose.watch-server = {
+  preHook = ''
+    # Cleanup on EXIT, this runs irrespective of exit-code of process-compose
+    trap "rm -rf ./data" EXIT
+    export USER=foo
+  '';
+};
+```
+
+### postHook
+
+Or if you'd like to run certain commands upon successful execution of `process-compose`, i.e exits with `exit-code: 0`, then add them to `postHook`:
+
+```nix
+process-compose.watch-server = {
+  postHook = ''
+    cat foo.txt
+  '';
+};
+```
+
 ## Module API
 
 Our submodule mirrors the [process-compose YAML schema](https://github.com/F1bonacc1/process-compose/blob/main/process-compose.yaml). A few things to remember: