docs: add README and env setup tool (#23)

Author: kt3k

.github/CONTRIBUTING.md

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+# deploy-cli
+
+The implementation of `deno deploy` subcommand.
+
+## How this works
+
+The source code of this repository is published to https://jsr.io/@deno/deploy
+
+`deno deploy` subcommand executes the main entrypoint of `jsr:@deno/deploy`
+[ref](https://github.com/denoland/deno/blob/efa4da8643c1ada18102bd3eeadb28171f7cdad6/cli/tools/deploy.rs#L47-L72).
+
+## How to develop
+
+Set the file url of main.ts of your local copy of this repository to
+`DENO_DEPLOY_CLI_SPECIFIER` env var. e.g.
+`DENO_DEPLOY_CLI_SPECIFIER=file:///path/to/deploy-cli/main.ts`, or run
+`source dev_env.sh` at the root of this repo (This sets
+`DENO_DEPLOY_CLI_SPECIFIER`)
+
+Then `deno deploy` subcommand uses your local copy as its implementation.

README.md

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+export DENO_DEPLOY_CLI_SPECIFIER="file://$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)/main.ts"