add readme

README.md

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+# Tiempo
+
+A [timetrap](https://github.com/samg/timetrap/) compatible time tracking
+command line application.
+
+## How to run
+
+You need [rust](https://rustup.rs), then clone the repo and simply run
+
+    cargo test
+
+to check that everything is working, and then
+
+    cargo build --release
+
+to have a binary at `target/release/tiempo` that you can then move to a
+directory in your `PATH`. Pro tip, alias it to `t`.
+
+Run
+
+    tiempo --help
+
+to see the options.