Add LDK calendar link

[ConorOkus]

https://calendar.google.com/calendar/u/0/r?cid=Y19lNmZ2NnZsc2hicG9vYjJtbWJ2Ymxra29qNEBncm91cC5jYWxlbmRhci5nb29nbGUuY29t

[tnull]

https://calendar.google.com/calendar/u/0/r?cid=Y19lNmZ2NnZsc2hicG9vYjJtbWJ2Ymxra29qNEBncm91cC5jYWxlbmRhci5nb29nbGUuY29t

In lightningdevkit/rust-lightning#1637 I decided to go with the 'embedded' link, as this one doesn't seem accessible without being logged in to Google.