Suggest maintained alternatives for Rental advisory (#1187)

* Suggest maintained alternatives for Rental advisory

* move `ouroboros` higher on the list as by far the most popular

* add `escher`; thanks to Nick12 for suggesting

Author: Shnatsel

crates/rental/RUSTSEC-2021-0134.md

@@ -13,3 +13,10 @@ unaffected = []
 # rental is unmaintained, author has moved on
 
 The author encourages users to explore other solutions, or maintain a fork.
+
+Maintained alternatives include:
+
+* [`ouroboros`](https://crates.io/crates/ouroboros)
+* [`recursive_reference`](https://crates.io/crates/recursive_reference)
+* [`fortify`](https://crates.io/crates/fortify)
+* [`escher`](https://crates.io/crates/escher)