README.md: release notes for new version

I will push the v0.13.0 tag immediately after this is submitted.

Change-Id: I15c68dad82d772c878af0ab8de1505ba76f7dd65
Reviewed-on: https://code-review.googlesource.com/16710
Reviewed-by: Ross Light <light@google.com>

README: changes for v0.12.0

Will tag the repo immediately after this is submitted.

Change-Id: I486ecd4496b9d4e1783ba2eed08b7159a14df282
Reviewed-on: https://code-review.googlesource.com/16070
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ross Light <light@google.com>

README: release notes for v0.11.0

I will tag the commit resulting from this CL with 'v0.11.0'.

Change-Id: Ia474ebf3d26f4308f9a8aa1caa1ba700035573ea
Reviewed-on: https://code-review.googlesource.com/15270
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Sarah Adams <shadams@google.com>

pubsub: add Subscription.Update; remove Subcription.ModifyPushConfig

BREAKING: ModifyPushConfig is removed.

Added Subcription.Update. Following the model used for GCS buckets,
defined a new type that parallels SubscriptionConfig, but has only
updateable fields, and can distinguish absence from a zero
value for each field.

Currently the push config is the only updateable field, and since
UpdateSubscription is not available to everyone yet, we call
ModifyPushConfig under the hood.

Change-Id: I00e27409ea4904bb4e416cd195883ce00b436f55
Reviewed-on: https://code-review.googlesource.com/14311
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Michael Darakananda <pongad@google.com>

bump version for breaking change

recent version to support dep tools

before pubsub client surface changes

beta release of logging, storage, datastore and bigquery

before ValueList removal and inference changes

prior to datastore client breakage