Clearly document a direct dependency on linkme is required

Author: dynaxis

README.md

@@ -14,7 +14,19 @@ With this crate, any trait object for a sub-trait of [`CastFrom`] can be cast di
 for another trait implemented by the underlying type if the target traits are registered beforehand
 with the macros provided by this crate.
 
+# Dependencies
+Add the following two dependencies to your `Cargo.toml`:
+
+```toml
+[dependencies]
+intertrait = "0.2"
+linkme = "0.2"
+```
+
+The `linkme` dependency is required due to the use of `linkme` macro in the output of `intertrait` macros.
+
 # Usage
+
 ```rust
 use intertrait::*;
 use intertrait::cast::*;