Add a section on community polyfills and tooling

Closes WICG#60. Closes WICG#108. We'll discuss in WICG#146 instead!

Co-Authored-By: Guy Bedford <guybedford@gmail.com>

README.md

@@ -51,6 +51,7 @@ _Or, how to control the behavior of JavaScript imports_
   - [Further implementation staging](#further-implementation-staging)
   - [`import:` URL loads and origins](#import-url-loads-and-origins)
   - [`import:` URL interaction with other loading infrastructure](#import-url-interaction-with-other-loading-infrastructure)
+- [Community polyfills and tooling](#community-polyfills-and-tooling)
 - [Acknowledgments](#acknowledgments)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
@@ -873,6 +874,20 @@ Several implementer questions come up around how `import:` URLs are envisioned t
 
 The high-level summary is that any fetch of an `import:` URL should be thought of as sugar for a series of if-then-else statements that in turn fetch the mapped-to URLs. For example, each fetch will pass through the service worker, until one succeeds.
 
+## Community polyfills and tooling
+
+Several members of the community have been working on polyfills and tooling related to import maps. Here are the ones we know about:
+
+* [@import-maps/generate](https://www.npmjs.com/package/@import-maps/generate) generates an import map from your `yarn.lock`.
+* [@jsenv/node-module-import-map](https://www.npmjs.com/package/@jsenv/node-module-import-map) generates an import map from your `package.json` and `node_modules/` directories.
+* [importly](https://www.npmjs.com/package/importly) generates an import map from a config file.
+* [Built-in Module Demo (with Rollup)](https://glitch.com/edit/#!/rollup-built-in-modules) contains code for a [Rollup](https://rollupjs.org/) plugin that generates an import map, focused on built-in module polyfills.
+* [SystemJS](https://github.com/systemjs/systemjs) provides a polyfill-like workflow for using import maps in older browsers with the System module format and `<script type="system-importmap">`.
+* [es-module-shims](https://github.com/guybedford/es-module-shims) provides an import maps polyfill for browsers with basic ES modules support.
+* [Deno](https://github.com/denoland/deno) is a JavaScript/TypeScript runtime with [built-in support for import maps](https://deno.land/manual.html#importmaps).
+
+Feel free to send a pull request with more! Also, you can use [#146](https://github.com/WICG/import-maps/issues/146) in the issue tracker for discussion about this space.
+
 ## Acknowledgments
 
 This document originated out of a day-long sprint involving [@domenic](https://github.com/domenic), [@hiroshige-g](https://github.com/hiroshige-g), [@justinfagnani](https://github.com/justinfagnani), [@MylesBorins](https://github.com/MylesBorins), and [@nyaxt](https://github.com/nyaxt). Since then, [@guybedford](https://github.com/guybedford) has been instrumental in prototyping and driving forward discussion on this proposal.