Minor tuning proposal on links related to CRS

Hello all

About the "Spatial Data on the Web Best Practices" draft document, I
would like to propose two minor editions on the links at the end of
chapter 7 (Coordinate Reference Systems) on
https://www.w3.org/TR/sdw-bp/#CRS-background. The document redirects the
reader to Spatial Reference (http://spatialreference.org/) and EPSG.io
(http://epsg.io/) web sites for CRS definitions. But the definitions on
those web sites sometime differ from the OGC and EPSG definitions
regarding axis order and units of measurement. The definitions on those
web sites are rather the ones used by GDAL (the library behind
MapServer, PostGIS and QGIS among others). Maybe the use of those
definitions was an intentional choice? (e.g. it was the choice of
GeoJSON before they deprecated CRS in the IETF standard, precisely
because they were not OGC/EPSG definitions). If the document rather aims
to reference OGC/EPSG definitions, then should the document links to the
authoritative source, which is http://epsg-registry.org/ ? It does not
mean to remove the other links, but maybe a little explanatory text like
below would be appropriate?

Instead of:

    Lists of CRS are available from the Open Geospatial Consotium CRS
    Register, Spatial Reference and EPSG.io (an open-source web service
    which simplifies discovery of coordinate reference systems utilized
    worldwide).

Maybe something like below?

    The authoritative source of CRS definitions is the EPSG registry.
    Those definitions are also available from the Open Geospatial
    Consotium CRS Register. Alternative source of CRS definitions are
    Spatial Reference and EPSG.io (an open-source web service which
    simplifies discovery of coordinate reference systems utilized
    worldwide). But those alternatives sometime uses different axis
    order and units of measurements than the authoritative definitions.

About the link to the Apache SIS documentation in the last paragraph
(thanks!), would you mind changing the
http://sis.staging.apache.org/book/en/developer-guide.html URL to
http://sis.apache.org/book/en/developer-guide.html (i.e. just remove the
"staging" part in the URL)?

    Best regards,

        Martin Desruisseaux