Update README.md

README.md

@@ -2,9 +2,9 @@
 
 A basic compiler based off of @thejameskyle's super-tiny-compiler, compiles a simple LISP-esque syntax into runnable JS.
 
-Currently supports a few built-ins, `add`, `subtract`, `assign` and `log`. Hopefully these are self-explanatory, or at least 
+Currently supports a few built-ins, `add`, `subtract`, `assign`, `def` and `log`. Hopefully these are self-explanatory, or at least 
 they should be from `example.mc`. A `#` denotes that the rest of the line (until the compiler sees `\n`) as a comment and 
-means that it will not be compiled.
+means that it will not be compiled, these may **not** be used within brackets (eg a function definition).
 
 The compiler runs like `node compiler.js file.mc` where `file.mc` is the file you wish to compile, and this will produce an
 `output.js` which requires `stdlib.js` to be in the same directory when running for now at least.