Incomplete hierarchy of structures in the README

[affeldt-aist]

https://github.com/math-comp/analysis/blob/master/README.md#mathematical-structures

does not display the mathematical structures of measure theory for example

[affeldt-aist]

nor the one of kernels and of measures which is actually the purpose of pending PRs, thus I re-milestone

[affeldt-aist]

We should maybe document this using HB.graph and the MathComp 2 branch.