Rollup merge of rust-lang#40213 - steveklabnik:extract-reference, r=alexcrichton

Move the reference into a submodule

Part of rust-lang#39588

This moves the reference out into its own submodule. Right now, it's under my own account; I would assume that it should go under `rust-lang`, but I guess there's an argument for the nursery too? I can fix that up before we commit it.

r? @alexcrichton @brson

Author: frewsxcv

.gitmodules

@@ -18,3 +18,6 @@
 [submodule "src/liblibc"]
 	path = src/liblibc
 	url = https://github.com/rust-lang/libc.git
+[submodule "reference"]
+	path = src/doc/reference
+	url = https://github.com/rust-lang-nursery/reference.git