converted ApiNotes Readme.txt to markdown

makes the apinotes Readme more readable on github

apinotes/README.md

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+# API Notes README
+
+API notes provide a mechanism by which Objective-C APIs can be
+annotated with additional semantic information not present within the
+original Objective-C headers. This semantic information can then be
+used by the Swift compiler when importing the corresponding Objective-C
+module to provide a better mapping of Objective-C APIs into Swift.
+
+API notes are organized into a set of `.apinotes` files. Each
+`.apinotes` file contains annotations for a single Objective-C module,
+written in YAML (FIXME: to be) described below. These YAML sources
+must be manually compiled into a binary representation (`.apinotesc`)
+that the Swift compiler will lazily load when it builds code, also
+described below. 
+
+# API Notes YAML Format
+
+TBD...
+
+# Compiling API notes
+
+The Swift compiler lazily loads API notes from compiled API notes
+files (`.apinotesc` files) and uses these annotations to affect the
+Swift signatures of imported Objective-C APIs. Compiled API notes
+files reside in the Swift module directory, i.e., the same directory
+where the `.swiftmodule` file would reside for the Swift overlay of
+that module. For system modules, the path depends on the platform
+and architecture
+
+Platform  | Path
+:------------- | :-------------
+  OSX | `$SWIFT_EXEC/lib/swift/macosx/`
+  iOS (32-bit) | `$SWIFT_EXEC/lib/swift/iphoneos/32`
+  iOS (64-bit) | `$SWIFT_EXEC/lib/swift/iphoneos`
+  iOS Simulator (32-bit) | `$SWIFT_EXEC/lib/swift/iphonesimulator/32`
+  iOS Simulator (64-bit) | `$SWIFT_EXEC/lib/swift/iphonesimulator`
+
+where `$SWIFT_EXEC/bin/swift` is the path to the Swift compiler
+executable.
+
+When updating API notes for a system module, recompile the API notes
+and place the result in the appropriate directories listed above. The
+Swift compiler itself need not be recompiled except in rare cases
+where the changes affect how the SDK overlays are built. To recompile
+API notes for a given module `$MODULE` and place them into their 
+
+### OSX
+```
+xcrun swift -apinotes -yaml-to-binary -target x86_64-apple-macosx10.10 -o $SWIFT_EXEC/lib/swift/macosx/$MODULE.apinotesc $MODULE.apinotes
+```
+
+### iOS (32-bit)
+```
+xcrun swift -apinotes -yaml-to-binary -target armv7-apple-ios7.0 -o $SWIFT_EXEC/lib/swift/iphoneos/32/$MODULE.apinotesc $MODULE.apinotes
+```
+
+### iOS (64-bit)
+```
+xcrun swift -apinotes -yaml-to-binary -target arm64-apple-ios7.0 -o $SWIFT_EXEC/lib/swift/iphoneos/$MODULE.apinotesc $MODULE.apinotes
+```
+
+### iOS Simulator (32-bit)
+```
+xcrun swift -apinotes -yaml-to-binary -target i386-apple-ios7.0 -o $SWIFT_EXEC/lib/swift/iphonesimulator/32/$MODULE.apinotesc $MODULE.apinotes
+```
+
+### iOS Simulator (64-bit)
+```
+xcrun swift -apinotes -yaml-to-binary -target x64_64-apple-ios7.0 -o $SWIFT_EXEC/lib/swift/iphonesimulator/$MODULE.apinotesc $MODULE.apinotes
+```
+
+To add API notes for a system module `$MODULE` that does not have them yet,
+create a new source file `$MODULE.apinotes`. Newly-added API notes will require
+re-running CMake. Updated API notes will be found by the build system during
+the next build.
+
+Note that Swift provides decompilation of binary API notes files via
+the `-apinotes -binary-to-yaml` option, which allows one to inspect
+the information the compiler is using internally. The order of the
+entities in the original YAML input is not preserved, so all entities
+in the resulting YAML output are sorted alphabetically.