Fixed use of ~/.../ directive in {src,test}/conditions.lisp, README.org

* src/conditions.lisp (incompatible-arguments): do not use "::" in
  package-qualified names for exported symbols
  (incompatible-initargs): likewise
  (print-object): likewise
* test/conditions.lisp (test suite maybe-print-cause-root): likewise
  (test suite maybe-print-explanation-root): likewise
  (mock-error/reference-condition): likewise
* README.org: likewise
Improved maybe-print-explanation in src/conditions.lisp

* src/conditions.lisp (maybe-print-explanation): at? parameter
  controls whether "." is printed when there is no explanation;
  updated documentation string accordingly
Added README.org as static-file in more-conditions.asd

* more-conditions.asd (system more-conditions): added README.org as
  static-file