add guideline to dissuade assertion abuse (#17)

README.md

@@ -84,3 +84,4 @@ Note that lines containing only comments are not considered empty lines.
 - When overloading a function from another module, the function name should be qualified with its module (e.g. `imported_function(...) = ...` is bad, `ParentModule.imported_function(...) = ...` is good).
 - Modules should only be loaded with `using`, never `import`. This helps prevent methods from being accidentally extended without qualification (whereas `import Module: f` allows `f` to be extended without qualification).
 - It's friendly to selectively `export` module bindings, but unfriendly to `export` *everything*. A module should `export` unambiguously named bindings that are part of the module's intended API. It's also okay to not export (but document) functions that are intended to be called in qualified form, e.g. `CSV.read`.
+- Do not use `@assert` statements for any error handling/input checking/etc. that should occur even in optimized builds. Per `@assert`'s documentation: "An assert might be disabled at various optimization levels."