document incremental build cache in builder docs

docs/03-github/04-builder.mdx

@@ -109,6 +109,7 @@ Builds can now be downloaded as Artifacts in the Actions tab.
 
 ## Caching
 
+### Library cache
 In order to make builds run faster, you can cache Library files from previous builds. To do so
 simply add Github Actions official [cache action](https://github.com/marketplace/actions/cache)
 before any unity steps.
@@ -127,6 +128,23 @@ Example:
 
 This simple addition could speed up your build by more than 50%.
 
+### Incremental build cache
+To further speedup consequent builds, you may want to cache the incremental build cache directory.
+
+Example:
+
+```yaml
+- uses: actions/cache@v3
+  with:
+    path: $HOME/work/_temp/_github_home/.cache/unity3d/bee
+    key: IncrementalBuildCache-MyProjectName-TargetPlatform
+    restore-keys: |
+      IncrementalBuildCache-MyProjectName-
+      IncrementalBuildCache-
+```
+
+On a windows runner (e.g. if building for il2cpp) this path is different, but the incremental build cache is automatically saved with the Library folder.
+
 ## Configuration options
 
 Below options can be specified under `with:` for the `unity-builder` action.