readme: add short introduction

README.md

@@ -1,4 +1,14 @@
-# mlibc is a C standard library
+# mlibc
+
+mlibc is a fully featured C standard library designed with portability in mind.
+
+We support a number of architectures (x86-64, i386, RISC-V, AArch64, m68k), and provide a clean syscall abstraction layer for new operating system ports to plug into.
+
+Unlike other portable C standard libraries like newlib, we aim for feature parity with glibc/musl i.e full pthread support and other GNU extensions.
+
+mlibc is [capable enough to run web browsers](https://forum.osdev.org/viewtopic.php?p=345952#p345952), though some features are still missing.
+
+Individual operating systems can opt in or out of certian features as desired; for example POSIX APIs like `pthread` are gated behind the POSIX 'option', Linux APIs like `epoll` are gated behind the Linux option, etc.
 
 ![Continuous Integration](https://github.com/managarm/mlibc/workflows/Continuous%20Integration/badge.svg)