[upstream_utils] Document adding a patch (NFC)

upstream_utils/README.md

@@ -54,7 +54,7 @@ Move the patch files to `upstream_utils`.
 mv *.patch allwpilib/upstream_utils/lib_patches
 ```
 
-Navigate back to `upstream_utils`
+Navigate back to `upstream_utils`.
 ```bash
 cd allwpilib/upstream_utils
 ```
@@ -64,3 +64,45 @@ Modify the version number in the call to `setup_upstream_repo()` in
 ```bash
 ./update_lib.py
 ```
+
+## Adding patch to thirdparty library
+
+The example below will add a new patch file to a hypothetical library called
+`lib`.
+
+Start in the `upstream_utils` folder. Restore the original repo.
+```bash
+./update_lib.py
+```
+
+Navigate to the repo.
+```bash
+cd /tmp/lib
+```
+
+Make a commit with the desired changes.
+```bash
+git add ...
+git commit -m "..."
+```
+
+Generate patch files.
+```bash
+git format-patch 2.0..HEAD --zero-commit --abbrev=40 --no-signature
+```
+where `2.0` is replaced with the version specified in `update_lib.py`.
+
+Move the patch files to `upstream_utils`.
+```
+mv *.patch allwpilib/upstream_utils/lib_patches
+```
+
+Navigate back to `upstream_utils`.
+```bash
+cd allwpilib/upstream_utils
+```
+
+Rerun `update_lib.py` to reimport the thirdparty files.
+```bash
+./update_lib.py
+```