Add readme.md

add usage

readme.md

@@ -0,0 +1,16 @@
+# calendar filter
+
+.ics calendar filter.
+
+## usage
+
+`
+/filtered-ics?url=the damn link&regex=valid regex to filter out
+`
+
+## how to run
+
+```sh
+git clone this repo
+cargo run
+```