add warning in INSTALL

Author: henrispriet

INSTALL.md

@@ -1,5 +1,9 @@
 # Installing from Source
 
+**Note: This document describes _building_ Rust _from source_.
+This is _not recommended_ if you don't know what you're doing.
+If you just want to install Rust, check out the [README.md](README.md) instead.**
+
 The Rust build system uses a Python script called `x.py` to build the compiler,
 which manages the bootstrapping process. It lives at the root of the project.
 It also uses a file named `config.toml` to determine various configuration