Add README.md

README.md

+# Web Instant
+
+Provides `Spot`, a version of `Instant` that works both on desktop and the web. On desktop, the internal representation uses `std::time::Instant`. On the web, the internal representation is an `f64`,
+and uses `js_sys::Date` methods to cover functinality where required
+
+## Install
+
+`cargo add web_instant`
+
+## Usage
+
+```
+use web_instant::Spot;
+use std::time::Duration;
+
+fn my_cross_platform_timer(last_time: Spot) {
+    let time_diff: Duration = Spot::now() - last_time;
+    println!("It has been {} seconds", time_diff.as_seconds());
+}
+```