set MSRV to 1.56.1 depending on tui v0.18.0

Cargo.toml

@@ -2,6 +2,7 @@
 name = "tui-textarea"
 version = "0.1.5"
 edition = "2021"
+rust-version = "1.56.1"
 authors = ["rhysd <lin90162@yahoo.co.jp>"]
 description = """
 tui-textarea is a simple yet powerful text editor widget for tui-rs. Multi-line

README.md

@@ -601,6 +601,10 @@ loop {
 
 See [`split` example](./examples/split.rs) and [`editor` example](./examples/editor.rs) for working example.
 
+## Minimum Supported Rust Version
+
+MSRV of this crate is depending on `tui` crate. Currently MSRV is 1.56.1.
+
 ## Versioning
 
 This crate is not reaching v1.0.0 yet. There is no plan to bump the major version for now. Current versioning policy is