add readme

Signed-off-by: Ignacio Hagopian <jsign.uy@gmail.com>

README.md

@@ -1,2 +1,26 @@
 # poseidon
-A Zig implementation of Poseidon hash function
+
+A Zig implementation of the Poseidon hash function, using the [Neptune optimizations](https://github.com/lurk-lab/neptune/blob/ef14a61b1aa7f8e92e6ace2190723c155e613a4a/spec/poseidon_spec.pdf).
+
+## Supported finite fields
+
+This implementation is currently targeting BN254 scalar field (i.e: BabyJubJub base field), to be compatible with:
+- [CircomLib](https://github.com/iden3/circomlib) repository.
+- [go-iden3-crypto](https://github.com/iden3/go-iden3-crypto/tree/master/poseidon) implementation.
+- [poseidon-rs](https://github.com/arnaucube/poseidon-rs) implementation.
+
+The parameters for BN254 were [pulled from CircomLib](https://github.com/iden3/circomlibjs/blob/4f094c5be05c1f0210924a3ab204d8fd8da69f49/src/poseidon_constants.json) which can be generated with the [official Sage script](https://extgit.iaik.tugraz.at/krypto/hadeshash) and transformed using a [CircomLibJS tool](https://github.com/iden3/circomlibjs/blob/main/tools/poseidon_optimize_constants.js) created by @jbaylina.
+
+Supporting other fields (e.g: BLS12-381 scalar field) would only involve generating the parameters.
+
+## Benchmarks
+
+To be included soon.
+
+## Future work
+
+Due to some limitations of JSON at `comptime`, the parameter parsing is dynamic for now. Whenever this gets fixed, we can avoid this (init) runtime overhead and strip down the binary size.
+
+## License
+
+MIT

src/poseidon.zig

@@ -11,7 +11,6 @@ pub fn Poseidon(comptime Fr: type, comptime w: u8) type {
             };
         }
 
-        // https://github.com/lurk-lab/neptune/blob/ef14a61b1aa7f8e92e6ace2190723c155e613a4a/spec/poseidon_spec.pdf
         pub fn hash(
             self: *@This(),
             input: [w - 1]Fr.MontgomeryDomainFieldElement,