Add Readme's to plugins and resources folders.

This will hopefully prevent people from messing with the installed files (and then getting confused when they upgrade) when they should be using the configuration (sub-)folder(s) instead. If I read cura-build correctly, just placing the files here will already copy them to the right directory, since that happens for all folders (and hopefully files) for those directories anyway.

plugins/README_plugins.txt

@@ -0,0 +1,11 @@
+This is the folder for _bundled_ plugins. If you place any other plugins here,
+they will not be seen or upgraded by any next version of Cura.
+
+If you want your (unbundled) plugins to work after an upgrade,
+please don't put them here, but install them either;
+ - via the Marketplace (recommended if available for that plugin),
+ - by dragging a `.curapackage` file onto Cura,
+ - or place it in the plugin folder in the configuration folder* for Cura.
+
+*) The plugin sub-folder of the configuration folder can be found by:
+   Open up Help -> Show Configuration Folder -> Navigate to the plugins folder.

resources/README_resources.txt

@@ -0,0 +1,11 @@
+This is the folder for _bundled_ resources. If you place, or tinker with,
+any resources here, they will not be seen or upgraded by next versions of Cura.
+
+If you want your (unbundled or altered) resources to work after an upgrade,
+please don't put them here, but install them either;
+ - for materials: via the Marketplace (recommended if avialable),
+ - by dragging a `.curapackage` file onto Cura,
+ - or place it in the right sub-folder in the configuration folder* for Cura.
+
+*) The configuration folder can be found by:
+   Open up Help -> Show Configuration Folder