README.md - RAM requirement on 32-bit *nix

Running `/usr/bin/time -v make` to build rust (using local llvm) shows the maximum memory usage at 715 megabytes on 32-bit x86 (on arm linux it's even less @ 580M).

Reworded according to @brson's input.

README.md

@@ -135,7 +135,7 @@ Snapshot binaries are currently built and tested on several platforms:
 You may find that other platforms work, but these are our officially
 supported build environments that are most likely to work.
 
-Rust currently needs about 1.5 GiB of RAM to build without swapping; if it hits
+Rust currently needs between 600MiB and 1.5GiB to build, depending on platform. If it hits
 swap, it will take a very long time to build.
 
 There is more advice about hacking on Rust in [CONTRIBUTING.md].