Document layer sources from geogratis.gc.ca(?)

[Malvoz]

Many examples have layer sources such as:

<mapml-viewer projection="CBMTILE" zoom="3" lat="62.7" lon="-90.3" controls>
  <layer- label="CBMT" src="https://geogratis.gc.ca/mapml/en/cbmtile/cbmt/" checked></layer->
  ...
</mapml-viewer>

Thus I think it's best to add some information on which layer sources are valid from geogratis.gc.ca for the different projections. It may also not be clear to users what it means, e.g. in legal terms, to use these resources. Probably best to clarify.

The introduction may be a good place for that, and/or #32.

[prushforth]

Each MapML document over on geogratis.gc.ca has a license link to the terms of use for that layer.

I think it might be instructive to use those examples in documentation of the <link rel="license" ...>. WDYT? IANAL.