Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Bug reports
+
+Use [the bug tracker](https://gitlab.com/pastebinrun/pastebinrun/issues).
+If it's a security issue, ensure "This issue is confidential and should
+only be visible to team members with at least Reporter access" is checked.
+
+# Development
+
+You need to have Node.js 12 and Rust 1.39 or higher installed. It's
+recommended to use [`rustup`](https://rustup.rs/) for managing Rust
+versions.
+
+1. Install Node.js packages.
+
+   ```sh
+   npm install
+   ```
+
+2. Run webpack. This will automatically rebuild JavaScript files on
+   every TypeScript change.
+
+   ```sh
+   node_modules/.bin/webpack --watch
+   ```
+
+3. Run the project. Ensure that `DATABASE_URL` is set to a new
+   PostgreSQL database. Migrations will automatically run on it.
+   If you want to test sandbox, you will also need to set up
+   `SANDBOX_URL` environment variable.
+
+   For instance, if you have created a `pastebinrun` database, you
+   can run pastebin.run as follows.
+
+   ```sh
+   env DATABASE_URL=postgresql:///pastebinrun cargo run
+   ```
+
+4. If a contribution adds new features, it's strongly encouraged
+   to add tests for those features.
+
+5. Reformat your code before submitting merge request. Use latest
+   Rust stable for reformatting the code.
+
+   ```sh
+   cargo fmt
+   ```