Add some more headers to installation documentations

In particular, I found that the disclaimer (promised in the second paragraph) doesn't stand out right now. I think the extra headers help with scanning this document, and making it easier to discern which steps are relevant to my particular flow through the options.

src/doc/trpl/installing-rust.md

@@ -41,6 +41,8 @@ $ sudo /usr/local/lib/rustlib/uninstall.sh
 If you used the Windows installer, just re-run the `.msi` and it will give you
 an uninstall option.
 
+## That disclaimer we promised
+
 Some people, and somewhat rightfully so, get very upset when we tell you to
 `curl | sh`. Basically, when you do this, you are trusting that the good
 people who maintain Rust aren't going to hack your computer and do bad things.
@@ -50,6 +52,8 @@ binary downloads][install-page].
 
 [from-source]: https://github.com/rust-lang/rust#building-from-source
 
+## Platform support
+
 Oh, we should also mention the officially supported platforms:
 
 * Windows (7, 8, Server 2008 R2)
@@ -66,6 +70,8 @@ integrated as the Linux/OS X experience is. We're working on it! If anything
 does not work, it is a bug. Please let us know if that happens. Each and every
 commit is tested against Windows just like any other platform.
 
+## After installation
+
 If you've got Rust installed, you can open up a shell, and type this:
 
 ```bash