docs: document the patches/<version>/ mechanism

Explain where out-of-tree patches live, when they are applied during
the build, and the workflow for adding a new patch via
`git format-patch`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Nikita Kalyazin <nikita.kalyazin@e2b.dev>

Author: kalyazin

README.md

@@ -34,6 +34,23 @@ and uploading the resulting binaries to a Google Cloud Storage (GCS) bucket.
   - On every push, GitHub Actions will automatically build the kernels and
     save it as an artifact.
 
+## Patches
+
+Out-of-tree patches are stored under `patches/<kernel-version>/` and
+applied during the build, after the source tree is reset to the kernel
+tag and the config is copied into place, but before `make olddefconfig`.
+Patches are applied in filename-sort order with `git apply`. Versions
+without a `patches/<version>/` directory are built unmodified.
+
+To add a new patch:
+```sh
+cd /path/to/linux
+git format-patch -1 --stdout <commit-sha> \
+  > /path/to/fc-kernels/patches/<kernel-version>/NNNN-<subject>.patch
+```
+Use the next free `NNNN` prefix in that version's directory and commit
+the patch file alongside any related changes.
+
 ## Architecture naming
 
 Output directories use Go's `runtime.GOARCH` convention (`amd64`, `arm64`)