Add some new files to the GIMP3-plug-in-porting-guide, including a README

devel-docs/GIMP3-plug-in-porting-guide/classes.md

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+# Useful Modules/Classes in GIMP 3.0+
+
+Here's a guide to the modules you're likely to need.
+It's a work in progress: feel free to add to it.
+
+Eventually we'll have online documentation for these classes.
+In the meantime, you can generate your own:
+```
+HTMLDOCDIR=/path/to/doc/dir
+g-ir-doc-tool -I /path/to/share/gir-1.0/ --language=Python -o $HTMLDOCDIR Gimp-3.0.gir
+```
+Then browse $HTMLDOCDIR with yelp, or generate HTML from it:
+```
+cd $HTMLDOCDIR
+yelp-build cache *.page
+yelp-build html .
+
+```
+
+You can also get some information in GIMP's Python console with
+*help(module)* or *help(object)*, and you can get a list of functions
+with *dir(object)*.
+
+## Gimp
+
+The base module: almost everything is under Gimp.
+
+## Gimp.Image
+
+The image object.
+
+Some operations that used to be PDB calls, like
+```
+pdb.gimp_selection_layer_alpha(layer)
+```
+are now in the Image object, e.g.
+```
+img.select_item(Gimp.ChannelOps.REPLACE, layer)
+```
+
+## Gimp.Layer
+
+The layer object.
+
+```
+fog = Gimp.Layer.new(image, name,
+                     drawable.width(), drawable.height(), type, opacity,
+                     Gimp.LayerMode.NORMAL)
+```
+
+## Gimp.Selection
+
+Selection operations that used to be in the PDB, e.g.
+```
+pdb.gimp_selection_none(img)
+```
+are now in the Gimp.Selection module, e.g.
+```
+Gimp.Selection.none(img)
+```
+
+## Gimp.ImageType
+
+A home for image types like RGBA, GRAY, etc:
+```
+Gimp.ImageType.RGBA_IMAGE
+```
+
+## Gimp.FillType
+
+e.g. Gimp.FillType.TRANSPARENT, Gimp.FillType.BACKGROUND
+
+## Gimp.ChannelOps
+
+The old channel op definitions in the gimpfu module, like
+```
+CHANNEL_OP_REPLACE
+```
+are now in their own module:
+
+```
+Gimp.ChannelOps.REPLACE
+```
+
+## Gimp.RGB
+
+In legacy plug-ins you could pass a simple list of integers, like (0, 0, 0).
+In 3.0+, create a Gimp.RGB object:
+
+```
+    c = Gimp.RGB()
+    c.set(240.0, 180.0, 70.0)
+```
+or
+```
+    c.r = 0
+    c.g = 0
+    c.b = 0
+    c.a = 1
+```