readme: added docs

README.md

@@ -6,3 +6,23 @@ to implement IEEE-754 floating point numbers. The audacity!
 The name is a mashup of `silly` and `IEEE-754`.
 
 WIP.
+
+## Installation
+
+Don’t. The makefile only includes a test target, because I want to test my code.
+
+## Usage
+
+```c
+silleee x;
+assilleee(x) = 1.0;
+
+silleee silleee_add(silleee, silleee); // addition
+silleee silleee_sub(silleee, silleee); // subtraction
+```
+
+That's it!
+
+<hr/>
+
+Have fun!