add SPAKE2+ (#628)

README.md

@@ -101,6 +101,7 @@ Contributions are welcome. Please take a quick look at the [contribution guideli
  * [radix](https://github.com/luislavena/radix) - Radix Tree implementation
  * [s2_cells](https://github.com/spider-gazelle/s2_cells) - [S2 Geometry](https://s2geometry.io/devguide/s2cell_hierarchy.html) for spatial indexing
  * [secure-remote-password](https://github.com/spider-gazelle/secure-remote-password) - SRP-6a protocol for authentication over an insecure network
+ * [SPAKE2+](https://github.com/spider-gazelle/SPAKE2_plus) - Password Authenticated Key Exchange (PAKE) protocol, comparable to SRP-6a
  * [splay_tree_map](https://github.com/wyhaines/splay_tree_map.cr) - Splay Tree implementation that conforms to the Hash ducktype
  * [verhoeff](https://github.com/spider-gazelle/verhoeff) - Implementation of the Verhoeff checksum algorithm