Add LICENSE and README

Author: keith

LICENSE

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+Copyright (c) 2021 Keith Smiley (http://keith.so)
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the 'Software'), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software is furnished to do so,
+subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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+# dyld-shared-cache-extractor
+
+[As of macOS Big Sur][mjtsai], instead of shipping the system libraries
+with macOS, Apple ships a generated cache of all built in dynamic
+libraries and excludes the originals. This tool allows you to extract
+these libraries from the cache for reverse engineering.
+
+## Usage
+
+Extract the default shared cache to `/tmp/libraries`:
+
+```bash
+dyld-shared-cache-extractor /System/Library/dyld/dyld_shared_cache_arm64e /tmp/libraries
+```
+
+## Installation
+
+```bash
+brew install dyld-shared-cache-extractor
+```
+
+## More details
+
+There are a few different ways you can interact with these shared
+caches.
+
+1. Depending on what you're doing inspecting them in [Hopper][hopper] is
+   the easiest option
+2. For a bit more functionality you can build the
+   `dyld_shared_cache_util` target from the latest `dyld` [source
+   dump][dump], but this requires some [modifications][modifications]
+
+The problem with the 2 options above is that they can lag behind format
+changes in the shared cache. This tool loads the private
+`dsc_extractor.bundle` from Xcode, meaning whichever it should always be
+able to extract the newest versions of the file for beta OS versions.
+
+This logic is based on the function at the bottom of
+`dyld3/shared-cache/dsc_extractor.cpp` from the `dyld` [source
+dump][dump].
+
+[dump]: https://opensource.apple.com
+[hopper]: https://www.hopperapp.com
+[mjtsai]: https://mjtsai.com/blog/2020/06/26/reverse-engineering-macos-11-0
+[modifications]: https://lapcatsoftware.com/articles/bigsur.html