Add caveat on PCH errors

Summary:
Moving files that have generated PCHs causes an error that you cannot clear easily.  Here are the instructions on how.

I was prompted to place this info here via #6797
Closes https://github.com/facebook/react-native/pull/7185

Differential Revision: D3453289

Pulled By: mkonicek

fbshipit-source-id: 8e16ea8f1bc3495209d1510a1caad2c6208c2e1e

docs/KnownIssues.md

@@ -67,6 +67,10 @@ React Native Android depends on [Fresco](https://github.com/facebook/fresco) for
 
 Try running `react-native init` with `--verbose` and see [#2797](https://github.com/facebook/react-native/issues/2797) for common causes.
 
+### Moving folder after iOS build
+
+Moving a React Native project after it has been compiled in iOS can sometimes cause errors with pre-compiled headers e.g. `mv MyAwesomeProject/ newDir/MyAwesomeProject`. This results in a message resembling: `error: PCH was compiled with module cache path ...`.  If you were compiling using Xcode you can [hold option when you clean the build](http://stackoverflow.com/a/15463219/3110982).  If you're using `react-native run-ios` you can manually clean the header cache by deleting the contents of your ModuleCache folder. e.g. `rm -rf ios/build/ModuleCache/*`
+
 ### Text Input Border
 
 The text input has by default a border at the bottom of its view. This border has its padding set by the background image provided by the system, and it cannot be changed. Solutions to avoid this is to either not set height explicitly, case in which the system will take care of displaying the border in the correct position, or to not display the border by setting underlineColorAndroid to transparent.