mention MSRV for bitvec

README.md

@@ -188,7 +188,9 @@ Some benchmarks are available on [Github](https://github.com/Geal/nom_benchmarks
 
 ## Rust version requirements
 
-The 6.0 series of nom requires **Rustc version 1.44 or greater** (compatible with 1.41.1 if building without the `alloc` or `std` features, ie `--no-default-features --features="regex,lexical"`).
+The 6.0 series of nom requires **Rustc version 1.44 or greater**, but it can change for some feature combinations:
+- compatible with 1.41.1 if building without the `alloc` or `std` features, ie `--no-default-features --features="regex,lexical"`
+- compatible with 1.51 if using the bitvec features
 
 Travis CI always has a build with a pinned version of Rustc matching the oldest supported Rust release.
 The current policy is that this will only be updated in the next major nom release.