Add the edition guide to doc.rust-lang.org

Author: steveklabnik

.gitmodules

@@ -64,4 +64,6 @@
 	path = src/tools/clang
 	url = https://github.com/rust-lang-nursery/clang.git
 	branch = rust-release-80-v1
-  
+[submodule "src/doc/edition-guide"]
+	path = src/doc/edition-guide
+	url = https://github.com/rust-lang-nursery/edition-guide

src/bootstrap/doc.rs

@@ -70,6 +70,7 @@ macro_rules! book {
 book!(
     Nomicon, "src/doc/nomicon", "nomicon";
     Reference, "src/doc/reference", "reference";
+    EditionGuide, "src/doc/edition-guide", "edition-guide";
     RustdocBook, "src/doc/rustdoc", "rustdoc";
     RustcBook, "src/doc/rustc", "rustc";
     RustByExample, "src/doc/rust-by-example", "rust-by-example";