Document check-merge-conflict argument 

Document --assume-in-merge in README

README.md

diff --git a/README.md b/README.md
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@@ -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.