Add a macros.clj to the project

[magomimmo]

It often happens that a CLJS lib needs macros. CLJS does not directly support macros and if a CLJS lib needs them, they are confined in a CLJ file which is compiled down by the CLJ compiler and the resulting namespace is made available to a CLJS code through the :require-macros namaspace directive in the CLJS calling namespace.

Most of the current CLJS libs put a macros.clj file in a CLJS source directory, which I don't like, because this breaks the separation of concerns principle.

Any macros.clj files should be put in a CLJ directory.

[magomimmo]

To support macros we do not need any special extension. Just document in the README.md file that if a lib requires macros and/or compile-time only functions, those have to be written in files hosted in the src/clj pathname.