Add a note for the 32 bit flag

Per #1375

Just add a note around the python script flag

Author: sbeckeriv

README.md

@@ -56,6 +56,8 @@ make install
 If, however, you have neither `rustc` nor `cargo` previously installed you can
 run:
 
+Note: if building for 32 bit systems run `BIT=32 python -B ..`
+
 ```sh
 python -B src/etc/install-deps.py
 ./configure --local-rust-root="$PWD"/rustc