change the README regarding linter.clj[c|s] files

mention `(in-ns 'joker.core)` and link to candid82#50

README.md

@@ -184,7 +184,13 @@ If you use `:refer :all` Joker won't be able to properly resolve symbols because
 (t/deftest ...)
 ```
 
-3. "Teach" Joker declarations from referred namespace. Joker executes the following files (if they exist) before linting your file: `.jokerd/linter.cljc` (for both Clojure and ClojureScript), `.jokerd/linter.clj` (Clojure only), `.jokerd/linter.cljs` (ClojureScript only). The rules for locating `.jokerd` directory are the same as for locating `.joker` file. So Joker can be made aware of any additional declarations (like `deftest` and `is`) by providing them in `.jokerd/linter.clj[s|c]` files. Note that such declarations become valid even if you don't require the namespace they come from, so this feature should be used sparingly (see [this discussion](https://github.com/candid82/joker/issues/52) for more details).
+3. "Teach" Joker declarations from referred namespace. Joker executes the following files (if they exist) before linting your file: `.jokerd/linter.cljc` (for both Clojure and ClojureScript), `.jokerd/linter.clj` (Clojure only), `.jokerd/linter.cljs` (ClojureScript only). The rules for locating `.jokerd` directory are the same as for locating `.joker` file.
+
+  > :warning: Joker can be made aware of any additional declarations (like `deftest` and `is`) by providing them in `.jokerd/linter.clj[s|c]` files. However, this means Joker cannot check that the symbols really are declared in your namespace, so this feature should be used sparingly.
+  >
+  > If you really want some symbols to be considered declared *in any namespace no matter what*, you can add `(in-ns 'joker.core)` to your `linter.clj[s|c]` and then declare those symbols.
+  >
+  > (see https://github.com/candid82/joker/issues/52 and https://github.com/candid82/joker/issues/50 for discussion). 
 
 I generally prefer first option for `clojure.test` namespace.