README.md: Update version-dependent features

Author: joshtriplett

README.md

@@ -40,20 +40,15 @@ libc = "0.2"
 
 * **deprecated**: `use_std` is deprecated, and is equivalent to `std`.
 
+* **deprecated**: `align` is deprecated (and always enabled).
+
 ## Rust version support
 
-The minimum supported Rust toolchain version is **Rust 1.13.0** . APIs requiring
+The minimum supported Rust toolchain version is **Rust 1.56.0** . APIs requiring
 newer Rust features are only available on newer Rust toolchains:
 
 | Feature              | Version |
 |----------------------|---------|
-| `union`              |  1.19.0 |
-| `const mem::size_of` |  1.24.0 |
-| `repr(align)`        |  1.25.0 |
-| `extra_traits`       |  1.25.0 |
-| `core::ffi::c_void`  |  1.30.0 |
-| `repr(packed(N))`    |  1.33.0 |
-| `cfg(target_vendor)` |  1.33.0 |
 | `const-extern-fn`    |  1.62.0 |
 
 ## Platform support