Add v3.8.8 Release notes

Note that in the attempt to release v3.8.7 before summer holidays,
v3.8.7 was already set in gerrit [1] but the tag nor the release were
ever finalised.

Gerrit version is now already set to v3.8.8-SNAPSHOT [2].

Version v3.8.7 is therefore withdrawn: next version will be v3.8.8

[1] https://gerrit-review.googlesource.com/c/gerrit/+/432140
[2] https://gerrit-review.googlesource.com/c/gerrit/+/432797

Change-Id: Ie6c0158db6b317a8675e7159b0e713bc27b4bb46
Release-Notes: skip