Update README devel instructions

Author: nikoskoukis-slamcore

README.md

@@ -151,7 +151,15 @@ this or other vim plugins
     the `vim` docstring (instead of manually updating the .txt file). See the
     misc/build_doc for more on how this is done.
 * Use [vader](https://github.com/junegunn/vader.vim) for unittesting. Use
-    `test/run-tests.sh` to run them.
+    `test/run-tests.sh` to run them. For this to run you have to first git clone
+    vim-repeat and vim-vader in the root of this repo:
+
+    ```sh
+    git clone https://github.com/junegunn/vader.vim.git
+    git clone https://github.com/tpope/vim-repeat.git
+    ```
+
+
 
 
 ## TODO