Fix example in README

[sueskind]

This MR fixes the example in the README.md file as it was unnecessarily complicated and did not actually compile/run. The following changes were made:

• Add Result<(), JobSchedulerError> as result type for main()
• Await the correct things (sched.add instead of Job::new)
• Add missing import for std::time::Duration
• Remove unused import tokio_cron_scheduler::JobToRun
• Replace info! and warn! macros with println! to avoid dependency to the log crate
• Remove usages of chrono to avoid the dependency
• Replace .unwrap()s with the more idiomatic ? operator
• Remove some examples that don't add any explanatory value
• Remove the feature flag introduced by this example, but add a disclaimer that the signal feature must be enabled for tokio-cron-scheduler
• Add explanatory comments
• Fix spelling/wording
• Formatting

Fixes #39 (and #37 to some extent).

[mvniekerk]

Thank you for the PR @sueskind !