Add README.

README.md

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+# A RPKI Relying Party in Rust
+
+## Getting Started (aka What’s that Rust thing you keep going on about?)
+
+If you don’t have it yet, you need Rust. There’s a tool called *rustup*
+for that. If you feel lucky, simply do:
+
+```bash
+curl https://sh.rustup.rs -sSf | sh
+```
+
+or get the file, have a look and then run it manually.
+
+Make sure `cargo` is in your path (*rustup* should tell you what to do),
+then, in the directory you cloned this repository to, say
+
+```bash
+cargo build
+```
+
+This will build the whole thing (or fail, of course). If it succeeds, you
+can run
+
+```bash
+cargo run
+```
+
+to run the binary that has been built. At this point, it will rsync all
+repository instances into `./test/repository` and validate them. You will
+need the `rsync` executable in your path.
+