Add draft description of the project.

README.md

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+# The Graphene
+
+Graphene is a library that implements an abstract graph. The class template allows
+creation of graphs of any types of nodes both directed and undirected with
+the custom weight function.
+
+## Shortest paths
+
+The library calculates shortest path between two nodes using the Dijkstra algorithm.
+However the term "shortest" is an abstraction, because the distance between
+to nodes is just a particular notion of the weight of the graph edge. `Graphene`
+allows to define custom weight functions, so that the algorithm will calculate
+not only the shortest path, but the path that corresponds to the minimal overall
+"weight".
+
+# See also
+
+[California Road Network](https://users.cs.utah.edu/~lifeifei/SpatialDataset.htm)