Move sections about contributing closer together

Author: jyn514

README.md

@@ -8,6 +8,9 @@ standard library, and documentation.
 [Rust]: https://www.rust-lang.org
 
 **Note: this README is for _users_ rather than _contributors_.**
+**Note: If you wish to _contribute_ to the compiler, you should read the
+[Getting Started][gettingstarted] of the rustc-dev-guide instead of this
+section.**
 
 ## Quick Start
 
@@ -18,10 +21,6 @@ Read ["Installation"] from [The Book].
 
 ## Installing from Source
 
-**Note: If you wish to _contribute_ to the compiler, you should read the
-[Getting Started][gettingstarted] of the rustc-dev-guide instead of this
-section.**
-
 The Rust build system uses a Python script called `x.py` to build the compiler,
 which manages the bootstrapping process. More information about it can be found
 by running `./x.py --help` or reading the [rustc dev guide][rustcguidebuild].