[tac_plus] plugin-loading: 3/3: more docs + sample plugins

[Steve Kemp]

  I've included the plugin README, and two sample plugins
 in the archive available here:

    http://steve.org.uk/plugins.tar.gz

  I imagine we'll not install these trivial plugins by default
 so they're reference/samples and we don't need to update
 the autoconf magic to do anything here.

Steve
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