Add a changelog entry.

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+New: The underlying type for types::global_dof_index used to be
+`unsigned long long int`. On all reasonable systems, this is actually
+a 64-bit unsigned integer type, but the name wouldn't tell you that
+unless you already knew. This is now fixed: Instead of the mouthful
+of a type, we now use the more concise `uint64_t`: An unsigned integer
+type with exactly 64 bits of accuracy.
+<br>
+Strictly speaking, this may be an incompatible change: On some
+systems, `unsigned long long int` and `uint64_t` may have the same
+size, but are not the same type. In practice, as long as you have used
+the type `types::global_dof_index`, you will not see any difference at
+all.
+<br>
+(Wolfgang Bangerth, 2020/03/01)