docs: update readme of crates with MSRV text (#5)

README.md

@@ -73,9 +73,18 @@ This crate has the following features:
   modules that require the standard library. If this feature is not set, `merge`
   is a `no_std` library.
 
-## Minimum Supported Rust Version
+## Minimum Rust version policy
 
-This crate supports Rust 1.64.0 or later.
+This crate's minimum supported `rustc` version is `1.64.0`.
+
+The current policy is that the minimum Rust version required to use this crate
+can be increased in minor version updates. For example, if `crate 1.0` requires
+Rust 1.20.0, then `crate 1.0.z` for all values of `z` will also require Rust
+1.20.0 or newer. However, `crate 1.y` for `y > 0` may require a newer minimum
+version of Rust.
+
+In general, this crate will be conservative with respect to the minimum
+supported version of Rust.
 
 ## Contact
 

crates/merge/README.md

@@ -73,9 +73,18 @@ This crate has the following features:
   modules that require the standard library. If this feature is not set, `merge`
   is a `no_std` library.
 
-## Minimum Supported Rust Version
+## Minimum Rust version policy
 
-This crate supports Rust 1.36.0 or later.
+This crate's minimum supported `rustc` version is `1.64.0`.
+
+The current policy is that the minimum Rust version required to use this crate
+can be increased in minor version updates. For example, if `crate 1.0` requires
+Rust 1.20.0, then `crate 1.0.z` for all values of `z` will also require Rust
+1.20.0 or newer. However, `crate 1.y` for `y > 0` may require a newer minimum
+version of Rust.
+
+In general, this crate will be conservative with respect to the minimum
+supported version of Rust.
 
 ## Contact
 

crates/merge_derive/README.md

@@ -8,4 +8,17 @@ SPDX-License-Identifier: CC0-1.0
 This crate provides a derive macro for the `merge::Merge` crate. See the
 [`merge`][`merge`] crate for more information.
 
+## Minimum Rust version policy
+
+This crate's minimum supported `rustc` version is `1.64.0`.
+
+The current policy is that the minimum Rust version required to use this crate
+can be increased in minor version updates. For example, if `crate 1.0` requires
+Rust 1.20.0, then `crate 1.0.z` for all values of `z` will also require Rust
+1.20.0 or newer. However, `crate 1.y` for `y > 0` may require a newer minimum
+version of Rust.
+
+In general, this crate will be conservative with respect to the minimum
+supported version of Rust.
+
 [`merge`]: https://lib.rs/crates/merge