add(rule): add the new git_diff_staged rule

README.md

@@ -159,6 +159,7 @@ using the matched rule and runs it. Rules enabled by default are as follows:
 * `java` – removes `.java` extension when running Java programs;
 * `git_add` – fix *"Did you forget to 'git add'?"*;
 * `git_checkout` – creates the branch before checking-out;
+* `git_diff_staged` – adds `--staged` to previous `git diff` with unexpected output;
 * `git_no_command` – fixes wrong git commands like `git brnch`;
 * `git_pull` – sets upstream before executing previous `git pull`;
 * `git_push` – adds `--set-upstream origin $branch` to previous failed `git push`;