Update README to reflect prng struct wrapper.

README.md

@@ -12,6 +12,7 @@ A C implementation of elliptic-curve-based Direct Anonymous Attestation signatur
 
 - The CMake build system is used for building.
 - gcc
+- libsodium >= 1.0.11 (optionally, see below)
 - For building the AMCL dependency:
   - python3
 
@@ -37,17 +38,21 @@ ctest -V
 ## Random number generator
 
 Many of the functions provided by this library (particularly, those used by an Issuer or a Member)
-require a pointer to a pseudo-random number generator (type `csprng`).
+require a pseudo random number generator (type `ecdaa_prng`).
 The security of these algorithms depends critically on the proper seeding of this prng.
 This means that the first use of any `ecdaa_prng` MUST be preceeded by a call to
-`ecdaa_prng_init` on the prng.
+`ecdaa_prng_init` (or `ecdaa_prng_init_custom`, see below) on the prng.
 
-The seed for the `ecdaa_prng` is generated using code adapted from Libsodium's
-`randombytes_buf`.
+In `ecdaa_prng_init`, the seed for the `ecdaa_prng` is generated from Libsodium's
+`randombytes_buf` function.
 A discussion on how this function works and any caveats can be found at Libsodium's webpage.
-Specifically, the procedure for using a non-standard random number generator
-(e.g. in environments where `/dev/urandom` isn't available but a hardware rng is)
-may be useful.
+
+To use a different function for obtaining cryptographically-secure random data for a seed,
+pass the option `-DDISABLE_LIBSODIUM_RNG_SEED_FUNCTION=ON` to cmake (this will remove the dependency on libsodium)
+and use the function `ecdaa_prng_init_custom` rather than `ecdaa_prng_init`,
+passing in a function pointer of type `ecdaa_prng_custom_seed_function_type` pointing
+to a function that is able to provide cryptographically-secure random bytes.
+Any initialization of the random source must be done prior to calling `ecdaa_prng_init_custom`.
 
 ## Naming Convention in API