readme: add small explanations

README.md

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+# Jitters
+
+This is a small experiment in writing a JIT-compiled calculator. It can
+pretty-print, evaluate the AST, compile to assembly, or JIT compile and
+interpret the input expression.
+
+## Building
+
+```sh
+./bootstrap # Also launches configure
+make
+```
+
+## How to use
+
+To show the different options, use:
+
+```sh
+./jitters --help
+```
+
+Once launched, you can write your expression in it's standard input.