README: mention to cargo update, mention rustup-toolchain-install-master

Author: RalfJung

README.md

@@ -55,7 +55,11 @@ Install Miri via `rustup`:
 rustup component add miri
 ```
 
-If `rustup` says the `miri` component is unavailable, that's because not all nightly releases come with all tools. Check out [this website](https://rust-lang.github.io/rustup-components-history) to determine a nightly version that comes with Miri and install that, e.g. using `rustup install nightly-2019-03-28`.
+If `rustup` says the `miri` component is unavailable, that's because not all
+nightly releases come with all tools. Check out
+[this website](https://rust-lang.github.io/rustup-components-history) to
+determine a nightly version that comes with Miri and install that, e.g. using
+`rustup install nightly-2019-03-28`.
 
 Now you can run your project in Miri:
 
@@ -131,7 +135,17 @@ able to just `cargo build` Miri.
 In case this fails, your nightly might be incompatible with Miri master.  The
 `rust-version` file contains the commit hash of rustc that Miri is currently
 tested against; you can use that to find a nightly that works or you might have
-to wait for the next nightly to get released.
+to wait for the next nightly to get released. You can also use
+[`rustup-toolchain-install-master`](https://github.com/kennytm/rustup-toolchain-install-master)
+to install that exact version of rustc as a toolchain:
+```
+rustup-toolchain-install-master $(cat rust-version) -c rust-src
+```
+
+Another common problem is outdated dependencies: Miri does not come with a
+lockfile (it cannot, due to how it gets embedded into the rustc build). So you
+have to run `cargo update` every now and then yourself to make sure you are
+using the latest versions of everything (which is what gets tested on CI).
 
 ### Testing the Miri driver
 [testing-miri]: #testing-the-miri-driver