Add more information to the error code for 'crate not found'

This comes up a lot when bootstrapping.

compiler/rustc_error_codes/src/error_codes/E0463.md

@@ -11,3 +11,24 @@ extern crate cake_is_a_lie; // error: can't find crate for `cake_is_a_lie`
 You need to link your code to the relevant crate in order to be able to use it
 (through Cargo or the `-L` option of rustc example). Plugins are crates as
 well, and you link to them the same way.
+
+## Common causes
+
+- The crate is not present at all. If using Cargo, add it to `[dependencies]`
+  in Cargo.toml.
+- The crate is present, but under a different name. If using Cargo, look for
+  `package = ` under `[dependencies]` in Cargo.toml.
+
+## Common causes for missing `std` or `core`
+
+- You are cross-compiling for a target which doesn't have `std` prepackaged.
+  Consider one of the following:
+  + Adding a pre-compiled version of std with `rustup target add`
+  + Building std from source with `cargo build -Z build-std`
+  + Using `#![no_std]` at the crate root, so you won't need `std` in the first
+    place.
+- You are developing the compiler itself and haven't built libstd from source.
+  You can usually build it with `x.py build library/std`. More information
+  about x.py is available in the [rustc-dev-guide].
+
+[rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html#building-the-compiler