Add rationale

Author: Ryan-rsm-McKenzie

README.md

@@ -1,3 +1,14 @@
 # `mmio`
 [![Main CI](https://img.shields.io/github/workflow/status/Ryan-rsm-McKenzie/mmio/Main%20CI?logo=github&style=flat)](https://github.com/Ryan-rsm-McKenzie/mmio/actions/workflows/main_ci.yml)
 [![Codecov](https://img.shields.io/codecov/c/github/Ryan-rsm-McKenzie/mmio?logo=codecov&logoColor=white&style=flat)](https://app.codecov.io/gh/Ryan-rsm-McKenzie/mmio)
+
+## What is `mmio`?
+`mmio` is a really simple, really small library for handling memory-mapped io on windows and linux.
+
+## Why not [`boost::iostreams::mapped_file`](https://www.boost.org/doc/libs/1_76_0/libs/iostreams/doc/classes/mapped_file.html)?
+A couple reasons:
+- It requires you to use Boost.
+- Passing unicode paths requires you to use `boost::filesystem`, which requires you to use _even more_ of Boost, and interops pretty terribly with `std::filesystem`.
+
+## Why not [`mio`](https://github.com/mandreyel/mio)?
+Being a header only library, it includes system headers into your code, and I don't enjoy fighting tooth and nail with `windows.h` for common identifiers.