docs: add liquid testnet api (Blockstream#417)

ChrisCho-H · web-flow · commit a5b223dc425f · 2022-11-22T00:34:07.000+02:00
diff --git a/API.md b/API.md
@@ -6,6 +6,7 @@ The blockstream.info public APIs are available at:
 - Bitcoin: https://blockstream.info/api/
 - Bitcoin Testnet: https://blockstream.info/testnet/api/
 - Liquid: https://blockstream.info/liquid/api/
+- Liquid Testnet: https://blockstream.info/liquidtestnet/api/
 
 For example:
 ```bash
