Update documentation

Author: josephlr

README.md

@@ -10,7 +10,7 @@
 A Rust library for retrieving random data from (operating) system source. It is
 assumed that system always provides high-quality cryptographically secure random
 data, ideally backed by hardware entropy sources. This crate derives its name
-from Linux's `getrandom` function, but is cross platform, roughly supporting
+from Linux's `getrandom` function, but is cross platform, aiming to support
 the same set of platforms as Rust's `std` lib.
 
 This is a low-level API. Most users should prefer using high-level random-number
@@ -40,7 +40,10 @@ fn get_random_buf() -> Result<[u8; 32], getrandom::Error> {
 
 ## Features
 
-This library is `no_std` compatible, but uses `std` on most platforms.
+This library is `no_std` for every supported target. However, getting randomness
+usually requires calling some external system API. This means most platforms
+will require linking against system libraries (i.e. `libc` for Unix,
+`Advapi32.dll` for Windows, Security framework on iOS, etc...).
 
 The `log` library is supported as an optional dependency. If enabled, error
 reporting will be improved on some platforms.