[docs] Update README.md

Author: olemorud

README.md

@@ -1,10 +1,12 @@
 # GCC problem matcher
 
+Unstable release!
+
 Creates annotations for warnings and errors in gcc builds.
 
 ## Inputs
 
-### root
+### build-directory
 
 **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.
 
@@ -15,5 +17,5 @@ Just add this line anywhere before running the build step.
 ```yaml
     - uses: olemorud/gcc-problem-matcher@master
       with:
-        root: /workspace/build
-```
+        build-directory: /workspace/build/
+```