Allow building as Apple Frameworks and XCFrameworks, including Simulator support

Author: dgovil

README.md

@@ -171,6 +171,37 @@ Or for visionOS:
 ```
 > python OpenUSD/build_scripts/build_usd.py --build-target visionOS --build-monolithic /path/to/my_usd_install_dir
 ```
+###### Framework Builds (Experimental)
+
+Builds for Apple platforms may optionally build as a framework using the `--build-apple-framework` flag. 
+
+**NOTE:** This feature is experimental and may change how it functions in future USD builds.
+
+- Framework builds are enabled by default for iOS and visionOS build targets. It can optionally be enabled for macOS.
+- Framework builds require monolithic builds. 
+- Building a universal macOS framework is currently not supported. Please generate the arches separately and `lipo` 
+  them together after.
+
+To add the Framework to your application, simply add `OpenUSD.framework` to your Xcode project.
+It is recommended to set it to `Embed and Sign`.
+
+When including sources from the Framework, you must prefix the include with the framework name.
+However, you should note that these includes are not portable to non-framework builds.
+e.g `#include <OpenUSD/pxr/pxr.h>`
+
+Optionally, if you want to avoid prefixed includes for your cross platform code, you can manually add the search path
+by setting `SYSTEM_HEADER_SEARCH_PATHS` in your Xcode target to the Headers directory within the framework.
+For example, if your Framework folder is in your project, under a folder with the same name as your target, you would
+set it as `$(SRCROOT)/$(TARGET_NAME)/OpenUSD.framework/Headers`. 
+
+OpenUSD also supports building a combined XCFramework as well of multiple targets.
+This command takes an optional list of targets to build, but will otherwise build all supported platforms.
+
+```
+> python OpenUSD/build_scripts/apple_utils.py xcframework /path/to/my_usd_install_dir
+```
+
+Note that currently, Simulator builds do not render with Imaging as Simulator targets have reduced Metal support.
 
 ##### Windows: