docs: update readme

README.md

@@ -1,6 +1,37 @@
 # ff-cl-gen [![Crates.io](https://img.shields.io/crates/v/ff-cl-gen.svg)](https://crates.io/crates/ff-cl-gen)
 
-OpenCL code generator for finite-field arithmetic over arbitrary prime fields.
+OpenCL code generator for finite-field arithmetic over prime fields constructed with Rust [ff](https://github.com/filecoin-project/ff) library.
+
+Notes:
+ - Limbs are 64-bit long.
+ - The library assumes that the most significant bit of your prime-field is unset. This allows for cheap reductions.
+
+Interface:
+
+```c
+typedef ulong limb;
+#define LIMB_BITS (64)
+
+#define FIELD_LIMBS ... // Number of 64bit limbs for this field
+#define FIELD_P ... // Normal form of field modulus
+#define FIELD_ONE ... // Montomery form of one
+#define FIELD_ZERO ... // Montomery/normal form of zero
+#define FIELD_BITS (FIELD_LIMBS * LIMB_BITS)
+
+typedef struct { limb val[FIELD_LIMBS]; } FIELD;
+
+bool FIELD_gte(FIELD a, FIELD b); // Greater than or equal
+bool FIELD_eq(FIELD a, FIELD b); // Equal
+FIELD FIELD_sub(FIELD a, FIELD b); // Modular subtraction
+FIELD FIELD_add(FIELD a, FIELD b); // Modular addition
+FIELD FIELD_mul(FIELD a, FIELD b); // Modular multiplication
+FIELD FIELD_sqr(FIELD a); // Modular squaring
+FIELD FIELD_double(FIELD a); // Modular doubling
+FIELD FIELD_pow(FIELD base, uint exponent); // Modular power
+FIELD FIELD_pow_lookup(global FIELD *bases, uint exponent); // Modular power with lookup table for bases
+bool FIELD_get_bit(FIELD l, uint i); // Get `i`th bit (From most significant digit)
+uint FIELD_get_bits(FIELD l, uint skip, uint window); // Get `window` consecutive bits, (Starting from `skip`th bit from most significant digit)
+```
 
 ## License