chore: added v0.19.x release notes

[devjgm]

This change is 

[coryan]

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @devjgm)

---

CHANGELOG.md, line 8 at r1 (raw file):

**BREAKING CHANGES:**
* refactor!: remove grpc_utils namespace; left aliases for backward compat (#158)

I think just this message is not enough context, maybe cut&paste the commit message (with some edits) into the release notes?

**POTENTIALLY BREAKING CHANGE:** Refactor the types and functions from
`google::cloud::grpc_utils` to `google::cloud::`. The old header files
and types should continue to work, as we kept aliases for them, but
there is some risk we missed something. The library name (the physical
.a and/or .so file) is not changed, the target names for CMake are not
changed. For Bazel, the old targets continue to work, but you might want
to move to newer targets that do not expose the backwards compatibility
headers.

Up to you.

[devjgm]

Yes, you're right. Of course. Updated. PTAL

[codecov]

Codecov Report

Merging #172 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #172   +/-   ##
=======================================
  Coverage   96.22%   96.22%           
=======================================
  Files          94       94           
  Lines        4395     4395           
=======================================
  Hits         4229     4229           
  Misses        166      166

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 498d64d...3741c21. Read the comment docs.

[coryan]

Additional nits on the release notes, but up to you.

Reviewed 1 of 1 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @devjgm)

---

CHANGELOG.md, line 14 at r2 (raw file):

  is not changed, the target names for CMake are not changed, the target name
  for Bazel moved to the new location.

That was true for the original commit, but no longer true after the additional changes for backwards compatibility, thus my edits. Consider:

... for CMake are not changed. For Bazel, the old targets continue to work, but
you might want to move to newer targets that do not expose the backwards
compatibility headers.

[devjgm]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @coryan)

---

CHANGELOG.md, line 14 at r2 (raw file):

for CMake are not changed. For Bazel, the old targets continue to work, but
you might want to move to newer targets that do not expose the backwards
compatibility headers

Sigh. I need to read more closely. Sorry. Accepted your edits.

[coryan]

Reviewed 1 of 1 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved