Add readme for embassy-time

embassy-time/README.md

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+# embassy-time
+
+Timekeeping, delays and timeouts.
+
+Timekeeping is done with elapsed time since system boot. Time is represented in
+ticks, where the tick rate is defined by the current driver, usually to match
+the tick rate of the hardware.
+
+Tick counts are 64 bits. At the highest supported tick rate of 1Mhz this supports
+representing time spans of up to ~584558 years, which is big enough for all practical
+purposes and allows not having to worry about overflows.
+
+[`Instant`] represents a given instant of time (relative to system boot), and [`Duration`]
+represents the duration of a span of time. They implement the math operations you'd expect,
+like addition and substraction.
+
+# Delays and timeouts
+
+[`Timer`] allows performing async delays. [`Ticker`] allows periodic delays without drifting over time.
+
+An implementation of the `embedded-hal` delay traits is provided by [`Delay`], for compatibility
+with libraries from the ecosystem.
+
+# Wall-clock time
+
+The `time` module deals exclusively with a monotonically increasing tick count.
+Therefore it has no direct support for wall-clock time ("real life" datetimes
+like `2021-08-24 13:33:21`).
+
+If persistence across reboots is not needed, support can be built on top of
+`embassy_time` by storing the offset between "seconds elapsed since boot"
+and "seconds since unix epoch".
+
+# Time driver
+
+The `time` module is backed by a global "time driver" specified at build time.
+Only one driver can be active in a program.
+
+All methods and structs transparently call into the active driver. This makes it
+possible for libraries to use `embassy_time` in a driver-agnostic way without
+requiring generic parameters.
+
+For more details, check the [`driver`] module.