Add a changelog entry.

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+Changed: The PolynomialSpace::compute_index() and
+PolynomialsP::directional_degree() functions used to return their
+information through an array that they received as a reference
+argument. Instead, they now return a `std::array<unsigned int,dim>`
+by value.
+<br>
+(Wolfgang Bangerth, 2018/05/09)