Add documentation on updating Cmake Headers

#18

This is a port of Vitor's change here WebKit/WebKit#7553.
We are adding instructions on how to add files to Cmake.

Author: stwrt

docs/Deep Dive/Build/AddingNewFile.md

@@ -18,7 +18,8 @@ To do that, turn on the target membership in respective framework as set the mem
 This will ensure the relevant header file is exported from WTF / WebCore to other downstream projects like WebKitLegacy.
 ![Screenshot of exporting a header file](../../assets/xcode-export-header.png)
 
-FIXME: Mention WTF_EXPORT_PRIVATE and WEBCORE_EXPORT.
-
-FIXME: Add instructions on how to add files to CMake.
+Non-cocoa ports, like WPE and GTK, use CMake for building the project. If the header is not platform-specific, you might 
+want to add an entry for it into the relevant Headers.cmake. For example, if a WebCore header is included by WebKit, you 
+will need to list the header in Source/WebCore/Headers.cmake.
 
+FIXME: Mention WTF_EXPORT_PRIVATE and WEBCORE_EXPORT.