doc: write README

Signed-off-by: Fabrizio Iannetti <fabrizio.iannetti@gmail.com>

README.md

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+# A calendar viewer in written in rust
+
+`calendar` is focused on displaying one or more files containing events in `ical` format
+
+it is currently in an early development phase.
+
+As a dependency, it requires the iced GUI repository to be cloned alongaside (see `Cargo.toml`)
+
+## supported views:
+
+* month view
+* year view
+
+## planned features
+
+* read ical events from a list of calendar files given as arguments
+* reload the events from a calendar file if the file changes
+* there is currently no plan to connect to a system calendar (e.g. evolution) or remote.