Add instructions to include custom files in the build env

Signed-off-by: Guillaume <[email protected]>

Author: gthvn1

README.md

@@ -35,6 +35,21 @@ Usage: ./build.sh {version_of_XCP_ng}
 ... where {version_of_XCP_ng} is a 'x.y' version such as 8.0.
 ```
 
+## Adding custom files
+
+In some cases, you may want to include additional files in the build
+environment - for example, a custom repository configuration.
+
+Suppose you have `/tmp/xcpng-users.repo` and you want it to be available
+as a YUM repository in the container. You can do this by adding the
+following line to your Dockerfile:
+```
+COPY /tmp/xcpng-users.repo /etc/yum.repos.d/
+```
+
+This approach is useful when you want to enable extra repositories during
+the build or runtime, especially when using `run.py` to manage repositories.
+
 ## Using the container
 
 Use the `run.py` script. It accepts a variety of parameters allowing for different uses: