Update README; add guides to docs

docs/guides/known-issues.md

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+# Known issues
+
+## Missing modules and Views
+
+This is an initial release of React Native for Web, therefore, not all of the
+views present on iOS/Android are released on Web. We are very much interested in
+the community's feedback on the next set of modules and views.
+
+Not all the modules or views for iOS/Android can be implemented on Web. In some
+cases it will be necessary to use a Web counterpart.
+
+## Missing component props
+
+There are properties that do not work across all platforms. All web-specific
+props are annotated with `(web)` in the documentaiton.
+
+## Platform parity
+
+There are some known issues in React Native where APIs could be made more
+consistent between platforms. For example, React Native for Web includes
+`ActivityIndicator` and a horizontal `ProgressBar`.
+
+Other parts of React Native, such as the `Animated` and `PanResponder` APIs,
+are highly complex and have not yet been ported to React Native for Web. Given
+the difficulties keeping these APIs in sync with React Native, we'd prefer the
+APIs to be published as separate npm packages. If not, we will consider a web
+implementation, possibly using the [Web Animations
+API/polyfill](https://github.com/web-animations/web-animations-js)