Add basic documentation page for Flow. Closes #57

Author: Daniel15

site/jekyll/guides/flow.md

@@ -0,0 +1,24 @@
+---
+layout: docs
+title: Flow
+---
+
+ReactJS.NET has support for stripping out [Flow](http://flowtype.org/) type
+annotations from your code. Flow is an open-source static type checker for
+JavaScript developed by Facebook. It adds static typing to JavaScript to improve
+developer productivity and code quality. You can learn more about Flow in
+[its release announcement](https://code.facebook.com/posts/1505962329687926/flow-a-new-static-type-checker-for-javascript/).
+
+How To Use It
+-------------
+Support for Flow is disabled by default. If you would like to use Flow type
+annotations, you must enable it in your site's configuration (normally
+`ReactConfig.cs`):
+
+```csharp
+ReactSiteConfiguration.Configuration.SetStripTypes(true);
+```
+
+This basically does the same thing as the offline transform tool mentioned in
+[the Flow documentation](http://flowtype.org/docs/running.html) so you do
+**not** need to run that tool manually.