warn of potential deprecations and schema updates

[crotwell]

Before releasing the documentation updates, would be good to review all issues that are "schema change" issues to create a list of future changes. Perhaps add warnings to any elements that are likely to change or be removed in the 2.0 revision.

[rcasey-earthscope]

Leaving a mark for Deprecation.

[crotwell]

Suggestion from @jschaeff to make this a separate document, so can update as the 2.0 review proceeds. Perhaps a github wiki page might be sufficient.

https://github.com/FDSN/StationXML/wiki/Potential-Changes-from-1.1-to-2.0

[crotwell]

moving this to "schema" label as we should add to wiki as 2.0 revision proceeds.

Perhaps consider moving wiki page into actual docs as "changes" on 2.0 release.

[crotwell]

Realized that we actually do already have this, section 1.4.2 Potential Future Changes, so question I guess is should we keep this section in the 1.1.1 docs, or move out?

See also #63