Document cargo features

Credit to @hinto-janai for adding such sections to their work on documenting
monero-serai in #568.

coins/monero/ringct/bulletproofs/README.md

@@ -4,3 +4,11 @@ Bulletproofs(+) range proofs, as defined by the Monero protocol.
 
 This library is usable under no-std when the `std` feature (on by default) is
 disabled.
+
+### Cargo Features
+
+- `std` (on by default): Enables `std` (and with it, more efficient internal
+  implementations).
+- `compile-time-generators` (on by default): Derives the generators at
+  compile-time so they don't need to be derived at runtime. This is recommended
+  if program size doesn't need to be kept minimal.