Merge pull request #840 from kairosswag/faq-rust-src

add faq entry for downloading rust source

Author: brson

README.md

@@ -555,6 +555,12 @@ rustup is the successor to [multirust]. rustup began as multirust-rs,
 a rewrite of multirust from shell script to Rust, by [Diggory Blake],
 and is now maintained by The Rust Project.
 
+### Can rustup download the Rust source code?
+
+The Rust source can be obtained by running `rustup component add rust-src`. 
+It will be downloaded to the `<toolchain root>/lib/rustlib/src/rust` 
+directory of the current toolchain.
+
 ### rustup fails with Windows error 32
 
 If rustup fails with Windows error 32, it may be due to antivirus