Squashed commit of the following:

commit a0ace30
Author: Adam Porter <adam@alphapapa.net>
Date:   Mon Nov 23 01:11:07 2020 -0600

    Docs: Fix TOC

commit bef9f3c
Author: Adam Porter <adam@alphapapa.net>
Date:   Mon Nov 23 01:09:33 2020 -0600

    Docs: (README.org) Add org-ql-defpred

commit ff02576
Author: Adam Porter <adam@alphapapa.net>
Date:   Mon Nov 23 00:52:06 2020 -0600

    Docs: Improve defpred.org

    Thanks to piyo in #emacsconf@freenode.

commit 18d6d73
Author: Adam Porter <adam@alphapapa.net>
Date:   Mon Nov 23 00:35:24 2020 -0600

    Docs: Improve custom predicate tutorial

    Closes #155.  Thanks to Merlin Göttlinger (@mgttlinger) for suggesting
    improvements.

commit b8cbe36
Author: Adam Porter <adam@alphapapa.net>
Date:   Mon Nov 23 00:05:52 2020 -0600

    Docs: Add custom predicate tutorial

commit b1008d0
Author: Adam Porter <adam@alphapapa.net>
Date:   Sun Nov 22 19:34:16 2020 -0600

    Docs: Update readme

commit 73d4f95
Merge: 9a2719f 148a193
Author: Adam Porter <adam@alphapapa.net>
Date:   Sun Nov 22 19:29:15 2020 -0600

    Merge branch 'wip/0.6-pre' into wip/define-predicate

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