Add docs for uvl-import

docs/plugins/uvl-import.md

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+The UVL import plugin enables the import of UVL files to CFM model.
+[Universal-Variability-Language](https://github.com/Universal-Variability-Language) is community-driven and aims to
+create a unified language for Variability Models.
+
+## Usage example
+
+Create an uvl file named `sandwich.uvl` containing a basic CFM:
+
+```text
+features
+    Sandwich cardinality [1]
+```
+
+The, import the model into the CFM toolbox and show some basic information about it:
+
+```bash
+python3 -m cfmtoolbox --import sandwich.uvl debug
+```
+
+## Limitation
+
+- UVL only supports two types of cardinalities: feature_cardinality and group_cardinality.
+  On the other hand CFM has three types of cardinalities, because the group_instance_cardinality is added.
+  The given group_cardinality is equal to the group_type_cardinality, even though some of the groups have a textual
+  representation instead of a numeric one (e.g. **or**) they can be converted.
+  The group_instance_cardinality has to be implied by the given type and internal structure of the model.
+- Additional attributes supplied in UVL are not supported by CFM, so will be ignored.
+- UVL has the capability to have more than one group as children for a feature, but CFM only allows children from the
+  same type.
+  This problem is solved by creating indexed subfeatures, which have the specific children and will be part of a parent,
+  that has very general cardinalities.
+  With that the model can be imported without any problems, altough you will have some information loss.
+
+```text
+features
+    Sandwich
+        alternative
+            Bread
+            Brioche
+        or
+            Sauce
+            Cheese
+```