Add link to https://github.com/spark/core-firmware/releases at bottom of changelog.md

Author: joegoggins

CHANGELOG.md

@@ -17,5 +17,7 @@
  - Wiring: `pinMode()` `INPUT` and `OUTPUT` constants had reversed values compared to Arduino. [#282](https://github.com/spark/core-firmware/issues/282)
  - Wiring: compiler error using `HEX` with `String`. [#210](https://github.com/spark/core-firmware/pull/210)
  - System Mode: MANUAL mode breaks OTA udpate [#294](https://github.com/spark/core-firmware/issues/294)
- 
- 
+
+## pre v0.3.3 versions
+
+See https://github.com/spark/core-firmware/releases