Add READMEs to each package's docs dir

Author: mjackson

packages/react-router-dom/docs/README.md

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+# React Router DOM Documentation Sources
+
+The documents in this directory are the sources for [React
+Router](https://reactrouter.com)'s documentation, specifically [the
+`react-router-dom` npm package](https://www.npmjs.com/package/react-router-dom).
+
+They were designed to be read online [on our
+website](https://reacttraining.com/react-router/), but that site is temporarily
+unavailable. While it is offline, you can browse the docs here on GitHub.
+
+- [API docs](/packages/react-router-dom/docs/api)
+- [Guides](/packages/react-router-dom/docs/guides)

packages/react-router-native/docs/README.md

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+# React Router Documentation Sources
+
+The documents in this directory are the sources for [React
+Router](https://reactrouter.com)'s documentation, specifically [the
+`react-router-native` npm
+package](https://www.npmjs.com/package/react-router-native).
+
+They were designed to be read online [on our
+website](https://reacttraining.com/react-router/), but that site is temporarily
+unavailable. While it is offline, you can browse the docs here on GitHub.
+
+- [API docs](/packages/react-router-native/docs/api)
+- [Guides](/packages/react-router-native/docs/guides)

packages/react-router/docs/README.md

@@ -0,0 +1,12 @@
+# React Router Documentation Sources
+
+The documents in this directory are the sources for [React
+Router](https://reactrouter.com)'s documentation, specifically [the
+`react-router` npm package](https://www.npmjs.com/package/react-router).
+
+They were designed to be read online [on our
+website](https://reacttraining.com/react-router/), but that site is temporarily
+unavailable. While it is offline, you can browse the docs here on GitHub.
+
+- [API docs](/packages/react-router/docs/api)
+- [Guides](/packages/react-router/docs/guides)