add tools/README.md (#4453)

tools/README.md

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+# Tools
+
+Documentation for various tooling in support of Deno development
+
+## docs.py
+
+This script is used to generate the API documentation for Deno. It can be useful
+to run locally to test the formatting of your changes to the documentation.
+
+If you would like to see how your JSDoc will be rendered after changing
+`cli/js/lib.deno.ns.d.ts`, you can run the following:
+
+First, make sure you have typedoc installed:
+
+```bash
+npm install typedoc --save-dev
+```
+
+Then run the doc generation tool:
+
+```bash
+./tools/docs.py
+```
+
+Output can be found in `./target/typedoc/index.html`
+
+## format.py
+
+This script will format the code (currently using prettier, yapf and rustfmt).
+It is a prerequisite to run this before code check in.
+
+To run formatting:
+
+```bash
+./tools/format.py
+```
+
+## lint.py
+
+This script will lint the code base (currently using eslint, pylint and clippy).
+It is a prerequisite to run this before code check in.
+
+To run linting:
+
+```bash
+./tools/lint.py
+```