Auto merge of #2414 - sbeckeriv:document-bit-env-flag-1375, r=alexcrichton

Per #1375

Just add a note around the python script BIT flag.

Author: bors

README.md

@@ -62,6 +62,7 @@ python -B src/etc/install-deps.py
 make
 make install
 ```
+Note: if building for 32 bit systems run `BITS=32 python -B ..`
 
 More options can be discovered through `./configure`, such as compiling cargo
 for more than one target. For example, if you'd like to compile both 32 and 64