pkglisten: add some docu

docs/pkglistgen.md

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+# The package list generator
+
+## Sources
+
+The package list generator consists of two scripts:
+
+- pkglistgen.py - python script using libsolv to do the hard work
+- pkglistgen.sh - shell script for the dirty work. It calls the product
+  converter locally and then splits the output. See existing scripts in script/
+  for examples how to call it for Rings, Stagings etc.
+
+## Input and Output
+
+There is one input package and two output packages:
+
+- 000package-groups: This is the input. It contains the *.product files, as
+  well as release package templates (e.g openSUSE-release.spec.in). The special
+  files groups.yml is read by pkglistgen.py to output group files that are
+  meant to be included by the product files.
+- 000product: This is the output container where resulting kiwi files are put
+- 000release-packages: This is the output container for release package spec files.
+
+## The groups.yml file
+
+The groups.yml is the input for the solver. It contains dictionaries with lists
+of packages. The special key OUTPUT contains a dictionary that lists the group
+files to generate.
+
+FIXME: continue here