added README

README.md

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+# chacha20
+A simple chacha20_poly1305 CLI encryption tool
+
+## Building
+Requires Rust and Cargo to build.
+Run `cargo build --release`, the binary will be built to `./target/release/chacha20`.
+
+### Testing
+Run `cargo test && cargo build && ./test.sh debug` to test the program.
+Alternatively, run `./test.sh` after building to test the release build's correctness.
+
+# Usage
+Copies stdin to stdout while encrypting or decrypting with the stream cipher.
+
+## Modes
+* Encrypt - Encrypt stdin to stdout
+* Decrypt - Decrypt stdin to stdout
+* Keygen - Generate a random key and IV and print them to stdout
+
+To see a more detailed explenation run `chacha20` with no arguments.
+
+## Example
+
+```shell
+$ echo "Hello world!" | chacha20 e 2>keys.cck > output.cc20
+$ chacha20 d $(cat keys.cck) < output.cc20
+Hello world!
+```
+
+# License
+GPL'd with <3