Update README.md with information about rustup component commands a…

…nd filters.

README.md

@@ -481,6 +481,45 @@ default toolchain:
 rustup default system
 ```
 
+## Working with components
+
+Components that enhance your default Rust toolchain, like `rustfmt`, can be
+installed like so:
+
+```
+rustup component add rustfmt
+```
+
+This will install `rustfmt` for the toolchain you have selected as your default
+using the `rustup toolchain default <toolchain>` command. If you want to see a list of the
+components that are available for your default toolchain, you can use:
+
+```
+rustup component list --filter available
+```
+
+This will list only the components that are available, without listing the
+components you have installed, or the ones that are required for your installed
+toolchain. If you want to filter by those particular statuses, the filter
+argument would look like:
+
+```
+rustup component list --filter required
+```
+
+or
+
+```
+rustup component list --filter installed
+```
+
+Required components are components that are required for you to compile Rust
+projects using the toolchain you have selected as your default. These are
+components like `rust-std`, `rust-doc`, `rustc`, and `cargo`. Installed
+components include required components, and are all components that you have
+installed. This includes non-critical components like `rustfmt` or `clippy`,
+which you may have installed previously.
+
 ## Working with custom toolchains and local builds
 
 For convenience of developers working on Rust itself, `rustup` can manage
@@ -530,9 +569,9 @@ If you need a more complex setup, rustup supports the convention used by
 the __curl__ program, documented in the ENVIRONMENT section of
 [its manual page][curlman].
 
-Note that some versions of `libcurl` apparently require you to drop the 
-`http://` or `https://` prefix in environment variables. For example, 
-`export http_proxy=proxy.example.com:1080` (and likewise for HTTPS). 
+Note that some versions of `libcurl` apparently require you to drop the
+`http://` or `https://` prefix in environment variables. For example,
+`export http_proxy=proxy.example.com:1080` (and likewise for HTTPS).
 If you are getting an SSL `unknown protocol` error from `rustup` via `libcurl`
 but the command-line `curl` command works fine, this may be the problem.