Add README section for committing back-end code

Signed-off-by: Vincent Petry <vincent@nextcloud.com>

README.md

@@ -56,8 +56,9 @@ Several apps that are included by default in regular releases such as [First run
 
 Otherwise, git checkouts can be handled the same as release archives, by using the `stable*` branches. Note they should never be used on production systems.
 
+### Working with front-end code 🏗
 
-### Building front-end code 🏗
+#### Building
 
 We are moving more and more towards using Vue.js in the frontend, starting with Settings. For building the code on changes, use these terminal commands in the root folder:
 
@@ -75,6 +76,8 @@ make watch-js
 make build-js-production
 ```
 
+#### Commiting changes
+
 **When making changes, also commit the compiled files!**
 
 We still use Handlebars templates some places in Files and Settings. We will replace these step-by-step with Vue.js, but in the meantime you need to compile them separately.
@@ -102,6 +105,17 @@ MODULE=user_status make build-js-production
 
 Please note that if you used `make build-js` or `make watch-js` before, you'll notice that a lot of files were marked as changed, so might need to clear the workspace first.
 
+### Working with back-end code 🏗
+
+When changing back-end PHP code, in general no additional steps are needed before checking in.
+
+However, if new files were created, you will need to run the following command to update the autoloader files:
+```bash
+build/autoloaderchecker.sh
+```
+
+After that, please also include the autoloader file changes in your commits.
+
 ### Tools we use 🛠
 
 - [👀 BrowserStack](https://browserstack.com) for cross-browser testing