Add basic specification of the default solver policy behaviour

doc/DefaultPolicy.md

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+# Default Solver Policy
+
+A solver policy defines behaviour variables of the dependency solver. It decides
+which versions are considered newer than others, which packages should be
+prefered over others and whether operations like downgrades or uninstall are
+allowed.
+
+## Selection of prefered Packages
+
+The following describe package pool situations with user requests and the
+resulting order in which the solver will try to install them.
+
+The rules are to be applied in the order of these descriptions.
+
+### Package versions
+
+Packages: Av1, Av2, Av3
+Installed: Av2
+
+Request: install A
+
+(Av3)
+
+### Repository priorities
+
+Packages Repo1.Av1, Repo2.Av1
+
+priority(Repo1) >= priority(Repo2) => (Repo1.Av1, Repo2.Av1)
+priority(Repo2) <  priority(Repo2) => (Repo2.Av1, Repo1.Av1)
+
+### Virtual Packages (provides)
+
+Packages Av1, Bv1
+Av1 provides Xv1
+Bv1 provides Xv1
+
+Request: install X
+
+priority(Av1.repo) >= priority(Bv1.repo) => (Av1, Bv1)
+priority(Av1.repo) < priority(Bv1.repo) => (Bv1, Av1)
+
+### Package replacements
+
+Packages: Av1, Bv2
+Bv2 replaces Av1
+
+Request: install A
+
+priority(Av1.repo) > priority(Bv2.repo) => (Av1, Bv2)
+priority(Av1.repo) = priority(Bv2.repo) => (Av1, Bv2)
+priority(Av1.repo) < priority(Bv2.repo) => (Bv2, Av1)
+
+Bv2.version is ignored, only the replacement version for A matters.