Add command to trigger git mergetool

Guides/vim.md

@@ -41,6 +41,11 @@ __vimdiff__ basics
 
 ### Configure git
 `git config --global diff.tool vimdiff`
+`git config --global merge.tool vimdiff`
+
+### Trigger conflict resolution
+
+`git mergetool`
 
 ### Make contents identical
 `dp` : diffput throws changement in the other window