SHA256 implementation note added in README.

doc/README.md

@@ -7,7 +7,11 @@
 | Additional Input       | No       |
 | Personalisation String | No       |
 
+* In C, a byte need not be 8 bits wide. However, this implementation uses the term 'byte' to refer to an 8-bit number.
+  Hence, fixed-width integer types are used liberally.
 * SHA-256 has been implemented from scratch, because I wanted this package to have no dependencies.
+  * However, if OpenSSL development libraries are found, its SHA-256 implementation is used if the C compiler provides
+    8-bit bytes.
 * `/dev/urandom` is read to obtain entropy for seeding and reseeding.
   * It is assumed to always provide sufficient entropy.
 * Nonces are generated by appending a monotonically increasing sequence number to the timestamp.
@@ -17,8 +21,6 @@
     which load the library at the same time will also generate the same nonce, because the sequence number is
     initialised to 0.) Which shouldn't be a problem, because their entropy inputs will be different with high
     probability.
-* In C, a byte need not be 8 bits wide. However, this implementation uses the term 'byte' to refer to an 8-bit number.
-  Hence, fixed-width integer types are used liberally.
 * The `hd` argument of any function (where applicable) denotes the HDRBG object to use.
   * If it is `NULL`, the internal HDRBG object is used.
   * For instance, `hdrbg_rand(NULL)` and `hdrbg_rand(foo)` are both valid invocations of `hdrbg_rand`—the former