Add tips to resolve conflicts with vimdiff

Guides/vim.md

@@ -52,3 +52,20 @@ __vimdiff__ basics
 
 `[c` previous
 
+### Resolving a conflict
+
+Three files : LOCAL | BASE | REMOTE
+
++--------------------------------+
+| LOCAL  |     BASE     | REMOTE |
++--------------------------------+
+|             MERGED             |
++--------------------------------+
+
+Three commands : `:diffget LO`, `:diffget BO`, `:diffget RE`
+
+### Unfold
+
+`zo`
+
+Useful to see more context.