READ-ADD: standardize boolean config options and help messages

[volkfox]

Currently we have some discrepancies between the documentation and command behavior:

1. Command summary still mentions `add-storage --read-add ' as a valid construct
2. Various documentation parts mention --read-add true and --read-add = true without distinction:

-a {true,false}, --read-add {true,false}

Storage location registered with --read-add=true must have write-access to allow new files to be added.

4. Built-in help does not specify that READ_ADD is a boolean flag:

ldb add-storage ~/Downloads/a1/ --read-add 
error: ldb add-storage: error: argument -a/--read-add: expected one argument

  -a READ_ADD, --read-add READ_ADD
                        Use this location for adding objects

We need to have a bit more consistency on these constructs.
Suggestions:

(1) Document that --read-add true or --read-add = true are the same and mutually replaceable (are they?)
(2) Update the built-in help
(3) For boolean arguments, assume TRUE to be the default, so --read-add is the same as --read-add true (doable?)