[docs] Update README

README.md

@@ -2,11 +2,18 @@
 
 Creates annotations for warnings and errors in gcc builds.
 
+## Inputs
 
-## Usage
+### root
 
-Just add a single line before running the build step.
+**Optional** Base directory for build. For builds done in a subdirectory, this should match that directory, otherwise the pattern match will not be able to point to the correct file.
+
+## Example usage
+
+Just add this line anywhere before running the build step.
 
 ```yaml
     - uses: olemorud/gcc-problem-matcher@master
-```
+      with:
+        root: /workspace/build
+```