docs: improve plugin developer's guide
docs: improve plugin developer's guide

Topics added: disabling lightly-used, experimental, or test plugins
by default. Discourage folks from creating bihash tables, processes
etc. from VLIB_INIT_FUNCTIONs.

Type: docs

Signed-off-by: Dave Barach <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@barachs.net>
Change-Id: I1235d64971e9ed50f992b75f96b77c934168276a

chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp