Schema doc updated

[Nick Barnes]

Yesterday I updated my online Bugzilla schema doc to reflect releases
2.18.4, 2.20, and 2.21.1.  After feedback on #mozwebtools I also added
a navigation table at the head of the document, to help you jump to
the table you need.

<http://www.ravenbrook.com/project/p4dti/tool/cgi/bugzilla-schema/>

Feedback to me, please.

I note for entertainment that we now (2.21.1) have 51 tables.

version #tables
2.0           7    
2.2           8
2.4           8
2.6          11
2.8          12
2.10         20
2.12         22
2.14         23
2.16         24
2.18         35
2.20         50
2.21.1       51

Thanks again to Perforce Inc for supporting this work, which is
incidental to the P4DTI Perforce/Bugzilla integration (because I need
to know the schema to interoperate with it).

Nick Barnes
P4DTI Project
Ravenbrook Limited