add README.md

Author: Techatrix

README.md

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+[![CI](https://github.com/allyourcodebase/mbedtls/actions/workflows/ci.yaml/badge.svg)](https://github.com/allyourcodebase/mbedtls/actions)
+
+# Mbed TLS
+
+This is [Mbed TLS](https://github.com/Mbed-TLS/mbedtls), packaged for [Zig](https://ziglang.org/).
+
+## Installation
+
+First, update your `build.zig.zon`:
+
+```
+# Initialize a `zig build` project if you haven't already
+zig init
+zig fetch --save git+https://github.com/allyourcodebase/mbedtls.git
+```
+
+You can then import `mbedtls` in your `build.zig` with:
+
+```zig
+const mbedtls_dependency = b.dependency("mbedtls", .{
+    .target = target,
+    .optimize = optimize,
+});
+your_exe.root_module.linkLibrary(mbedtls_dependency.artifact("mbedtls"));
+```