Improve README / man pages

[pitag-ha]

At the moment, the README refers to dune-release help release. What do you think about copying the content of that to the README?

Furthermore, I found the following part in  dune-release help release a bit confusing: "WRITE THE RELEASE NOTES        Carefully write the release notes in the package's change log, these        are essential time savers for users of the package."

When I saw that, I thought maybe I had to overwrite the changelog file with specific release notes for that release. So I think it might be better to clarify that part a bit:

• the changelog top bit (everything before the last release header) will be used as release notes. So when using dune-release, the changelog should be in the format one wants the release notes to be in, in particular it should start with a header of the version of the new release (that part was also not clear to me).
• if all relevant changes have already been listed in the changelog, there's nothing else to take into account. Only if the changelog is not up-to-date you have to update it or if there's no changelog at all, you have to write the release notes into the changelog file.