Add Sponder to RSS feed customization Toolbox (#123)

Sponder is an online RSS feed transformation service that allows users to build custom processing flows with keyword/regex filtering and feed merging. It works with any RSS reader.

Author: kristjan

README.md

@@ -1476,6 +1476,7 @@ Inspired by [Awesome lists](https://github.com/sindresorhus/awesome) and [@realS
 - [feedless](https://feedless.org/getting-started) <sup>[1340](https://t.me/s/aboutrss/1340)</sup> [![Open-Source Software][oss icon]](https://github.com/damoeb/feedless)
 - [rss-tools](https://adulau.github.io/rss-tools/): A set of crappy Python scripts to handle RSS in an Unix way [![Open-Source Software][oss icon]](https://github.com/adulau/rss-tools/)
 - [RSSBrew](https://github.com/yinan-c/RSSBrew) <sup>[1465](https://t.me/s/aboutrss/1465)</sup> [![Open-Source Software][oss icon]](https://github.com/yinan-c/RSSBrew)
+- [Sponder](https://sponder.app/): Build custom RSS processing flows with keyword/regex filtering and feed merging; works with any RSS reader. ![Online][Online icon]
 
 ### Full Article Extractors