Add some documentation

README.md

@@ -0,0 +1,17 @@
+go-and
+======
+
+Fast bitwise and for `[]byte` slices.
+
+```go
+import "github.com/bwesterb/go-and"
+
+func main() {
+    var a, b, dst []byte
+
+    // ...
+
+    // Sets dst to the bitwise and of a and b
+    and.And(dst, a, b)
+}
+```

lib.go

@@ -1,9 +1,13 @@
+// and implements bitwise and for two byte-slices.
 package and
 
 import (
 	"encoding/binary"
 )
 
+// Writes bitwise and of a and b to dst.
+//
+// Panics if len(a) ≠ len(b), or len(dst) ≠ len(a).
 func And(dst, a, b []byte) {
 	if len(a) != len(b) || len(b) != len(dst) {
 		panic("lengths of a, b and dst must be equal")