Add readme.

README.md

@@ -1 +1,46 @@
-# pathfinder2
+## Pathfinder2
+
+Pathfinder is a collection of tools related to
+computing transitive transfers in the
+[CirclesUBI](https://joincircles.net) trust graph.
+
+### Building
+
+This is a rust project, so assuming `cargo` is installed, `cargo build`
+creates two binaries: The server (default) and the cli.
+
+Both need a file that contains the trust graph edges to work.
+A reasonably up to date edge database file can be obtained from
+https://chriseth.github.io/pathfinder2/edges.dat
+
+
+#### Using the Server
+
+`cargo run --release <port>` will start a JSON-RPC server listening on the given port.
+
+It implements the interface specified in https://hackmd.io/Gg04t7gjQKeDW2Q6Jchp0Q
+
+It has two performance parameters that are currently hardcoded in the source:
+
+Number of worker threads: 4
+
+Size of request queue: 10
+
+#### Using the CLI
+
+The CLI will load an edge database file and compute the transitive transfers
+from one source to one destination. You can limit the number of hops to explore
+and the maximum amount of circles to transfer.
+
+
+The options are:
+
+`cargo run --release --bin cli <from> <to> [<max_hops> [<max_amount>]] <edges.dat>] [--dot <dotfile>]`
+
+For example 
+
+`cargo run --release --bin cli 0x9BA1Bcd88E99d6E1E03252A70A63FEa83Bf1208c 0x42cEDde51198D1773590311E2A340DC06B24cB37 edges.dat 3 1000000000000000000`
+
+Computes a transfer of at most `1000000000000000000`, exploring 3 hops.
+
+If you specify `--dot <dotfile>`, a graphviz/dot representation of the transfer graph is written to the given file.