In this commit, we add a new folder within the `docs` directory that will
house the release notes for all versions of `lnd` going forward. Moving to
use `git` for the release notes rather than the normal drafting procedure
lets us keep track of the set of changes more consistency over time, lets
contributors add their release notes over time, and also means we'll no
longer need to do a crunch at the tail end of the release process to fill in
the set of release notes.
For the 0.13.1 release notes, I've done a scan and added anything that has
been merged since the last major version tag. In a follow up change, we'll
add a new Github action that fails the build if an entry in the release
notes linking to the newly added PR isn't committed as well. Until then, any
PRs merged in should ensure that they add an entry within the relevant
milestone.
