Merge pull request #3418 from krishnakumarg1984/kg/add_appendixe_url

In Appendix D, adds hyperlink to Appendix E

src/appendix-04-useful-development-tools.md

@@ -95,7 +95,7 @@ fn main() {
 The `for` loop variable is now named `_i`, and the warning no longer appears.
 
 You can also use the `cargo fix` command to transition your code between
-different Rust editions. Editions are covered in Appendix E.
+different Rust editions. Editions are covered in [Appendix E][editions].
 
 ### More Lints with Clippy
 
@@ -178,3 +178,4 @@ particular IDE. Your IDE will gain abilities such as autocompletion, jump to
 definition, and inline errors.
 
 [rust-analyzer]: https://rust-analyzer.github.io
+[editions]: appendix-05-editions.md