docs: add CONTRIBUTING.md

Author: kt3k

.github/CONTRIBUTING.md

@@ -0,0 +1,9 @@
+# Development
+
+Set `DENO_DEPLOY_CLI_SPECIFIER` env var to `main.ts` file url on your local machine.
+
+```
+export DENO_DEPLOY_CLI_SPECIFIER=file:///path/to/deploy-cli/main.ts
+```
+
+and run `deno deploy` commands, which uses your local copy of deploy-cli.