Add Elixir SDK to SDKs table

[e-fu]

We just published ZenHive/mpp — an Elixir implementation of MPP, available on Hex as {:mpp, "~> 0.1.0"}.

It implements the core protocol (Challenge, Credential, Receipt, Headers, Errors) and Stripe as the first payment method, with Plug middleware for Phoenix/Plug apps.

Would be great to be added to the SDKs table in the README:

| [ZenHive/mpp](https://github.com/ZenHive/mpp) | Elixir |

Happy to open a PR instead if preferred.