Merge pull request #1410 from LuaAndC/doc-plugins-multi

plugins/README: clarify how to enable many plugins

plugins/README.md

@@ -16,7 +16,12 @@ MXE_PLUGIN_DIRS=foo/ make libpng
 ```
 
 If needed, you can also pass multiple directories by separating them with a
-space: `MXE_PLUGIN_DIRS='foo1/ foo2/'`.
+space: `MXE_PLUGIN_DIRS='foo1/ foo2/'`. A plugin takes effect only if it is
+provided in `MXE_PLUGIN_DIRS`. If you run `make` multiple times, do not
+remove a plugin path from `MXE_PLUGIN_DIRS`, otherwise MXE will rebuild
+without the plugin. For example, if you want to build qbittorrent from
+`apps` plugin with gcc 6 provided by `gcc6` plugin, set `MXE_PLUGIN_DIRS`
+to 'plugins/gcc6 plugins/apps' and then run `make qbittorrent`.
 
 For details on `*.mk` contents, please consult the contents of this directory
 and `src/*.mk`.