Document check-merge-conflict argument 

Document --assume-in-merge in README
diff --git a/README.md b/README.md
index 7368831..2c8cf4f 100644
--- a/README.md
+++ b/README.md
@@ -57,6 +57,7 @@
 
 #### `check-merge-conflict`
 Check for files that contain merge conflict strings.
+  - `--assume-in-merge` - Allows running the hook when there is no ongoing merge operation
 
 #### `check-shebang-scripts-are-executable`
 Checks that scripts with shebangs are executable.