update README.md for specifying msrv

Author: suyashb95

README.md

@@ -147,6 +147,30 @@ lints can be configured and the meaning of the variables.
 To deactivate the “for further information visit *lint-link*” message you can
 define the `CLIPPY_DISABLE_DOCS_LINKS` environment variable.
 
+### Specifying the minimum supported Rust version
+
+Projects that intend to support old versions of Rust can disable lints pertaining to newer features by
+specifying the minimum supported Rust version (msrv) in the clippy configuration file.
+
+```toml
+msrv = "1.30.0"
+```
+
+The msrv can also be specified as an inner attribute, like below.
+
+```rust
+#![feature(custom_inner_attributes)]
+#![clippy::msrv = "1.30.0"]
+
+fn main() {
+  ...
+}
+```
+
+Tilde/Caret version requirements(like `^1.0` or `~1.2`) can be specified as well.
+
+Note: `custom_inner_attributes` is an unstable feature so it has to be enabled explicitly.
+
 ### Allowing/denying lints
 
 You can add options to your code to `allow`/`warn`/`deny` Clippy lints: