doc: SIMD is enabled automatically

This was done in #490, but the documentation wasn't updated.

Closes #491

README.md

@@ -191,13 +191,7 @@ assert!(matches.matched(6));
 
 ### Usage: enable SIMD optimizations
 
-This crate provides an `unstable` feature that can only be enabled on nightly
-Rust. When this feature is enabled, the regex crate will use SIMD optimizations
-if your CPU supports them. No additional compile time flags are required; the
-regex crate will detect your CPU support at runtime.
-
-When `std::arch` becomes stable, then these optimizations will be enabled
-automatically.
+SIMD optimizations are enabled automatically on Rust stable 1.27 and newer.
 
 
 ### Usage: a regular expression parser

build.rs

@@ -15,13 +15,9 @@ fn main() {
 }
 
 fn enable_simd_optimizations(version: &str) {
-    // If we're using nightly Rust, then we can enable vector optimizations.
-    // Note that these aren't actually activated unless the `unstable` feature
-    // is enabled.
-    //
-    // We also don't activate these if we've explicitly disabled auto
-    // optimizations. Disabling auto optimizations is intended for use in
-    // tests, so that we can reliably test fallback implementations.
+    // We don't activate SIMD optimizations these if we've explicitly disabled
+    // them. Disabling auto optimizations is intended for use in tests, so that
+    // we can reliably test fallback implementations.
     if env::var_os("CARGO_CFG_REGEX_DISABLE_AUTO_OPTIMIZATIONS").is_some() {
         return;
     }