Update Exercises Directory Names to Reflect Order

exercises/16_lifetimes/README.md

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+# Lifetimes
+
+Lifetimes tell the compiler how to check whether references live long
+enough to be valid in any given situation. For example lifetimes say
+"make sure parameter 'a' lives as long as parameter 'b' so that the return
+value is valid".
+
+They are only necessary on borrows, i.e. references,
+since copied parameters or moves are owned in their scope and cannot
+be referenced outside. Lifetimes mean that calling code of e.g. functions
+can be checked to make sure their arguments are valid. Lifetimes are
+restrictive of their callers.
+
+If you'd like to learn more about lifetime annotations, the
+[lifetimekata](https://tfpk.github.io/lifetimekata/) project
+has a similar style of exercises to Rustlings, but is all about
+learning to write lifetime annotations.
+
+## Further information
+
+- [Lifetimes (in Rust By Example)](https://doc.rust-lang.org/stable/rust-by-example/scope/lifetime.html)
+- [Validating References with Lifetimes](https://doc.rust-lang.org/book/ch10-03-lifetime-syntax.html)