Document rust_1_27 feature

Author: vks

README.md

@@ -118,6 +118,7 @@ optional features are available:
 - `i128_support` enables support for generating `u128` and `i128` values.
 - `log` enables some logging via the `log` crate.
 - `nightly` enables all unstable features (`i128_support`, `simd_support`).
+- `rust_1_27` enables `RangeInclusive` support (requiring Rust >= 1.27).
 - `serde1` enables serialization for some types, via Serde version 1.
 - `simd_support` enables uniform sampling of SIMD types (integers and floats).
 - `stdweb` enables support for `OsRng` on `wasm32-unknown-unknown` via `stdweb`