README: add initial readme

Signed-off-by: Sean Cross <sean@xobs.io>

README.md

@@ -0,0 +1,31 @@
+# The Engine Assembler
+
+Provides an assembler-as-macro that lets you assemble The Engine bytecode
+in your program. For example, to create an infinite loop, run:
+
+```rust
+    let mcode = engine25519_as::assemble_engine25519!(
+        start:
+            brz start, #0
+    );
+```
+
+## Disassembly
+
+You can use a basic disassembler provided by this crate. To disassemble a single opcode, run:
+
+```rust
+    let opcode = engine25519_as::disasm::Opcode::from_i32(0x14_7985);
+    println!("Opcode: {}", opcode);
+```
+
+## Debugging the Macro
+
+To debug the macro, use the `nightly` toolchain and enable the `macro-debug` feature.
+
+An easy way to do this is:
+
+```sh
+$ cargo +nightly test --features macro-debug
+...
+```