docs: add a table for alternative implementations (#229)

Motivation:
#84 (comment)

Note this PR doesn't describe the status of "feature parity" b/c I'm not
sure what status that would be for nmt-rs. Seems like we can add that
later.

README.md

@@ -98,14 +98,18 @@ Where `nid_0 = nid_1 = 0` and `nid_2 = nid_3 = 1` and `data_i = "leaf_i"` for `i
 
 ## Related
 
-This implementation was heavily inspired by the initial implementation in the celestiaorg [prototype].
+This implementation was heavily inspired by the initial implementation in [celestiaorg/lazyledger-prototype](https://github.com/celestiaorg/lazyledger-prototype).
+
+Non-endorsed implementations of NMT exist in other languages:
+
+Language | Repo
+---------|------------------------------------------------------------------
+Rust     | [Sovereign-Labs/nmt-rs](https://github.com/Sovereign-Labs/nmt-rs)
 
 <!--- TODO references --->
 [academic paper]: https://arxiv.org/abs/1905.09274
 [@musalbas]: https://github.com/musalbas
 
-[prototype]: https://github.com/celestiaorg/lazyledger-prototype
-
 ## Contributing
 
 Markdown files must conform to [GitHub Flavored Markdown](https://github.github.com/gfm/). Markdown must be formatted with: