Introduce recommendations for external specification modules

[thiagowfx]

Bug: #506, #587

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[thiagowfx]

@OrKoN I'm not quite sure what we really want to add here...

(from the discussions on w3c/permissions#425)

Thoughts?

(Corresponding WG meeting discussion: #587 (comment))

[OrKoN]

I think it is not what we want. Let me come back to this tomorrow.

[jgraham]

There are two different things here:

• Open specifications, which can in principle define whatever they want. However we can suggest they they should avoid extending existing commands or modules where possible (i.e. prefer to add new modules; for extending existing modules updating the source specification of the existing module is preferred), and never define names including a colon.
• Implementations. For implementations we want to define that they must not add additional modules or extend new modules unless the added modules, commands, events, or properties have a correctly prefixed name (i.e. one that contains a :).

[thiagowfx]

Previously:

When defining [=extension modules=], implementers should consider the
possibility of future standardization of the module. If the module is
likely to be standardized in the future, implementers should use a
[=module name=] that does not contain a colon ":" character.

[OrKoN]

LGTM