Rollup merge of #24032 - steveklabnik:beta_install, r=alexcrichton

 Default is now beta, add nightly in the unstable section

src/doc/trpl/installing-rust.md

@@ -18,9 +18,9 @@ $ sudo sh rustup.sh
 ```
 
 If you're on Windows, please download either the [32-bit
-installer](https://static.rust-lang.org/dist/rust-nightly-i686-pc-windows-gnu.exe)
+installer](https://static.rust-lang.org/dist/rust-1.0.0-beta-i686-pc-windows-gnu.exe)
 or the [64-bit
-installer](https://static.rust-lang.org/dist/rust-nightly-x86_64-pc-windows-gnu.exe)
+installer](https://static.rust-lang.org/dist/rust-1.0.0-beta-x86_64-pc-windows-gnu.exe)
 and run it.
 
 If you decide you don't want Rust anymore, we'll be a bit sad, but that's okay.

src/doc/trpl/unstable.md

 % Unstable Rust
+
+Rust provides three distribution channels for Rust: nightly, beta, and stable.
+Unstable features are only available on nightly Rust. For more details on this
+process, see [this post](http://blog.rust-lang.org/2014/10/30/Stability.html).
+
+To install nightly Rust, you can use `rustup.sh`:
+
+```bash
+$ curl -s https://static.rust-lang.org/rustup.sh | sudo sh -s -- --channel=nightly
+```
+
+If you're concerned about the [potential insecurity](http://curlpipesh.tumblr.com/) of using `curl | sudo sh`,
+please keep reading and see our disclaimer below. And feel free to use a two-step version of the installation and examine our installation script:
+
+```bash
+$ curl -f -L https://static.rust-lang.org/rustup.sh -O
+$ sudo sh rustup.sh --channel=nightly
+```
+
+If you're on Windows, please download either the [32-bit
+installer](https://static.rust-lang.org/dist/rust-nightly-i686-pc-windows-gnu.exe)
+or the [64-bit
+installer](https://static.rust-lang.org/dist/rust-nightly-x86_64-pc-windows-gnu.exe)
+and run it.
+
+If you decide you don't want Rust anymore, we'll be a bit sad, but that's okay.
+Not every programming language is great for everyone. Just run the uninstall
+script:
+
+```bash
+$ sudo /usr/local/lib/rustlib/uninstall.sh
+```
+
+If you used the Windows installer, just re-run the `.exe` and it will give you
+an uninstall option.
+
+You can re-run this script any time you want to update Rust. Which, at this
+point, is often. Rust is still pre-1.0, and so people assume that you're using
+a very recent Rust.
+
+This brings me to one other point: some people, and somewhat rightfully so, get
+very upset when we tell you to `curl | sudo sh`. And they should be! 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. That's a good instinct!
+If you're one of those people, please check out the documentation on [building
+Rust from Source](https://github.com/rust-lang/rust#building-from-source), or
+[the official binary downloads](http://www.rust-lang.org/install.html). And we
+promise that this method will not be the way to install Rust forever: it's just
+the easiest way to keep people updated while Rust is in its alpha state.
+