docs(README): add note on required Rust versions

Author: hawkw

README.md

@@ -6,3 +6,13 @@ ALARM: Another Library for Allocating and Releasing Memory
 ALARM ~is~ _will be_ the new [SOS](https://github.com/hawkw/sos-kernel) memory allocator.
 
 **NOTE:** ALARM is currently _very_ early in the development process and most functionality has yet to be implemented.
+
+
+Building ALARM
+--------------
+
+Building ALARM requires the [nightly Rust compiler](https://doc.rust-lang.org/book/first-edition/release-channels.html), which you can install using [`rustup`](https://www.rustup.rs/). 
+
+Although some components of ALARM may also be compatible with the stable Rust compiler, the core functionality of this library as a memory allocator introduces a hard dependency on the [`allocator_api`](https://github.com/rust-lang/rfcs/blob/master/text/1398-kinds-of-allocators.md) language feature, currently only available on the nightly compiler.
+
+Continuous integration [builds](https://travis-ci.org/hawkw/alarm) of ALARM run against the _latest_ nightly compiler, so compatibility with older nightlies is not always assured.