Tweak docs

README.md

@@ -41,7 +41,7 @@ If you don't need them, disabling the default features may reduce code size and
 portable-atomic = { version = "1", default-features = false }
 ```
 
-If your crate supports no-std environment and requires atomic CAS, enabling the `require-cas` feature will allow the `portable-atomic` to display helpful error messages to users on targets requiring additional action on the user side to provide atomic CAS.
+If your crate supports no-std environment and requires atomic CAS, enabling the `require-cas` feature will allow the `portable-atomic` to display a [helpful error message](https://github.com/taiki-e/portable-atomic/pull/100) to users on targets requiring additional action on the user side to provide atomic CAS.
 
 ```toml
 [dependencies]

src/lib.rs

@@ -35,7 +35,7 @@ If you don't need them, disabling the default features may reduce code size and
 portable-atomic = { version = "1", default-features = false }
 ```
 
-If your crate supports no-std environment and requires atomic CAS, enabling the `require-cas` feature will allow the `portable-atomic` to display helpful error messages to users on targets requiring additional action on the user side to provide atomic CAS.
+If your crate supports no-std environment and requires atomic CAS, enabling the `require-cas` feature will allow the `portable-atomic` to display a [helpful error message](https://github.com/taiki-e/portable-atomic/pull/100) to users on targets requiring additional action on the user side to provide atomic CAS.
 
 ```toml
 [dependencies]