Update doc G"

README.md

 
 1. Run `git commit`
 2. Run `git cat-file commit HEAD`
-3. Modify the `catfile_text`, 
+3. Modify the `catfile_text` and `commit_msg` in `main()` , also modify the hash condition `if(unlikely(*(uint16_t *)hashbuf == 0))`. 
+4. build and run
+5. Run `git reset HEAD~1` and then commit with the result.