Convert "README.rst" to "README.md" (#63)

Co-authored-by: Arnold Hanser <[email protected]>

Author: igt-ahanser

README.md

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+# CPython Apple source dependencies
+
+A meta-package for building the binary packages for iOS, tvOS, watchOS, and
+visionOS that a CPython build requires. This includes:
+
+- BZip2
+- XZ
+- libFFI
+- mpdecimal
+- OpenSSL 1.1
+- OpenSSL 3.0
+
+The repository works by downloading, patching, and building binaries for each
+SDK target and architecture that is requried. The compiled library is packed
+into a tarball for distribution in "installed" form - that is, the contents of
+the `include` and `lib` folders are included.
+
+The binaries support arm64 for iOS, appleTV and visionOS devices, and arm64_32
+for watchOS. They also supports device simulators on both x86_64 and arm64
+hardware, except that visionOS is officially unsupported by Apple on x86_64.
+This should enable the code to run on:
+
+- **iOS 13.0 or later, on**:
+  - iPhone (6s or later)
+  - iPad (5th gen or later)
+  - iPad Air (all models)
+  - iPad Mini (2 or later)
+  - iPad Pro (all models)
+  - iPod Touch (7th gen or later)
+- **macOS 11 or later, using Mac Catalyst (14.2 or later), on**:
+  - MacBook (2015 or later)
+  - MacBook Air (2013 or later)
+  - MacBook Pro (Late 2013 or later)
+  - Mac mini (2014 or later)
+  - iMac (2014 or later)
+  - iMac Pro (2017 or later)
+  - Mac Pro (2013 or later)
+  - Mac Studio
+- **tvOS 9.0 or later, on**:
+  - Apple TV (4th gen or later)
+- **watchOS 4.0 or later, on**:
+  - Apple Watch (4th gen or later)
+- **visionOS 2.0 or later, on**:
+  - Apple Vision Pro (all models)
+
+## Quickstart
+
+**Unless you're trying to directly link against one of these libraries, you
+don't need to use this repository**. When you obtain a CPython build for an
+Apple platform, it will have already been linked using these libraries.
+
+If you *do* need to link against these libraries for some reason, you can use
+the pre-compiled versions that are published on the
+[Github releases page](https://github.com/beeware/cpython-apple-source-deps/releases).
+You don't need to compile them yourself.
+
+However, if you *do* need to compile your own version for some reason:
+
+- Clone this repository
+- Create a Python 3 virtual environment. Any supported Python 3 release should
+  be sufficient; some tools require python3 as part of their build.
+- In the root directory, run:
+  - `make` (or `make all`) to build everything.
+  - `make iOS` to build everything for iOS.
+  - `make BZip2` to build BZip2 for every platform
+  - `make BZip2-iOS` to build BZip2 for iOS
+
+This should:
+
+1. Download the original source packages
+2. Patch them as required for compatibility with the selected OS
+3. Build the libraries for the selected OS and architecture
+4. Package a tarball containing build products.
+
+The resulting artefacts will be packaged as `.tar.gz` files in the `dist`
+folder.