Improve contributing.md

Author: chenglou

CONTRIBUTING.md

@@ -25,13 +25,13 @@ Thanks for your interest. Below is an informal spec of how the plugin's server c
 ## Install Dependencies
 
 - Run `npm install` at the root. This will also install the npm modules for both the `client` and `server` folders.
-- `opam switch 4.12.0`. OPAM [here](https://opam.ocaml.org). This is needed for the `analysis` folder, which is native code.
+- `opam switch 4.12.0` (if you haven't created the switch, do it). OPAM [here](https://opam.ocaml.org). This is needed for the `analysis` folder, which is native code.
 - Optionally, you can `opam install ocamlformat` and format the `.ml` files in `analysis`.
 
 ## Build & Run
 
 - `npm run compile`. You don't need this if you're developing this repo in VSCode. The compilation happens automatically in the background.
-- `cd analysis && make`.
+- `cd analysis && make depend && make` (you only need `make depend` for the first run).
 
 ## Test