Re-introduce the introduction to tBTC in README
I somehow lost this during the MD->AsciiDoc conversion of the README.
@@ -22,6 +22,7 @@ toc::[]
== Getting started
+* Read the link:./docs/introduction-to-tbtc.md[introduction to tBTC].
* Read the http://docs.keep.network/tbtc/[specification].
* Consult http://docs.keep.network/tbtc/solidity/[Solidity API
documentation].
