Plugin Compile Documentation

[rgleason]

IMHO We do not have very good documentation about the need to
download from sourceforge and extract opencpn-buildwin.zip to the source\buildwin directory.

1. No README in plugin main git repository that I know of mentions it other than to mention installing necessary particular binaries to buildwin.

2. When executing "cmake -T v120_xp .. could there be a message that the "SVG dlls are missing from source\buildwin. Extract opencpn-buildwin.zip to source\buildwin"?

3. If the plugin is compiled with no svg dlls in buildwin, it will compile and install and all plugins loaded after will not install.(confirmed once) Possibly in some cases the plugin will install but Opencpn will not run or may crash when clicking the plugin icon (unconfirmed).

New plugin developers would appreciate a notice in the first "Cmake -T v120_xp .." command if possible.

Also the plugin documentation does not cover this newer step!! so that should be amended.

[rgleason]

Mike R advises that i have this wrong. If the files are in place in MSVC then they are not needed in the plugin buildwin!
After some trials I beliewe he is correct.

[rgleason]

Mike explained that those dlls are somehow incorporated in the file opencpn.lib, made by MSVC, so the plugin will need a version of opencpn.lib that has the dlls etc incorporated in order to run properly.