README: document basic features

README.md

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+# Introduction
+
+A library for making state machines :
+ - no dynamic allocation needed
+ - all states will share a common interface, which may then be used to interact
+ with states
+ - transitionning between states is a safe operation (memory owne by the former
+ state is still valid when constructing the new one)
+ - a list of allowed transitions can be used to restrict what state changes are
+ allowed
+ - one may log state transitions
+ - states are reusable
+
+# Usage
+
+See `test.cc`
+
+# Limitations
+
+ - When transitionning from a state to another, a deduction guide must be given
+ for the checker to know from which state the transition comes from. It may be
+ "fixed" by introducing a state identifier, which is super easy but adds a
+ performance overhead for little apparent benefit. Introducing a proxy class
+ which has friendly access for proxying transitions to the automaton sort of
+ solves the problem, but makes the syntax heavier.
+ - transitionning to a new state while already constructing a state is undefined
+ behavior
+
+# FAQ
+
+`is it production ready ?`
+No, it's a toy, the almost untested™ kind
+
+`Where's the build system ?`
+That's like two header files, what would you like to build ?