docs(contribute): add explainations about SQL migration (#893)

# Description

This pull request adds some explanations on how to do the SQL
migrations, required to contribute to the project, when required.

CONTRIBUTING.md

@@ -18,6 +18,13 @@ Dyad is an Electron app.
 npm install
 ```
 
+**Apply migrations:**
+
+```sh
+npm run db:generate
+npm run db:push
+```
+
 **Run locally:**
 
 ```sh