docs: expand on developer environment setup (#3208)

I just walked through setting up the development environment. I fixed a few small typos, but also tried to add my notes for what I saved in `.zshrc`. I figure most contributors are doing a similar thing?

CONTRIBUTING.md

@@ -129,10 +129,13 @@ project's service account.
 (e.g. doorway-cliff-677) for the Firestore project.
 - `GCLOUD_TESTS_GOLANG_FIRESTORE_KEY`: The path to the JSON key file of the
 Firestore project's service account.
+- `GCLOUD_TESTS_API_KEY`: API key for using the Translate API created above.
+
+As part of the setup that follows, the following variables will be configured:
+
 - `GCLOUD_TESTS_GOLANG_KEYRING`: The full name of the keyring for the tests,
 in the form
 "projects/P/locations/L/keyRings/R". The creation of this is described below.
-- `GCLOUD_TESTS_API_KEY`: API key for using the Translate API.
 - `GCLOUD_TESTS_GOLANG_ZONE`: Compute Engine zone.
 
 Install the [gcloud command-line tool][gcloudcli] to your machine and use it to
@@ -159,7 +162,7 @@ $ gsutil acl ch -g cloud-logs@google.com:O gs://$GCLOUD_TESTS_GOLANG_PROJECT_ID
 # Creates a PubSub topic for integration tests of storage notifications.
 $ gcloud beta pubsub topics create go-storage-notification-test
 # Next, go to the Pub/Sub dashboard in GCP console. Authorize the user
-# "service-<numberic project id>@gs-project-accounts.iam.gserviceaccount.com"
+# "service-<numeric project id>@gs-project-accounts.iam.gserviceaccount.com"
 # as a publisher to that topic.
 
 # Creates a Spanner instance for the spanner integration tests.
@@ -178,7 +181,38 @@ $ gcloud kms keys create key2 --keyring $MY_KEYRING --location $MY_LOCATION --pu
 # Sets the GCLOUD_TESTS_GOLANG_KEYRING environment variable.
 $ export GCLOUD_TESTS_GOLANG_KEYRING=projects/$GCLOUD_TESTS_GOLANG_PROJECT_ID/locations/$MY_LOCATION/keyRings/$MY_KEYRING
 # Authorizes Google Cloud Storage to encrypt and decrypt using key1.
-gsutil kms authorize -p $GCLOUD_TESTS_GOLANG_PROJECT_ID -k $GCLOUD_TESTS_GOLANG_KEYRING/cryptoKeys/key1
+$ gsutil kms authorize -p $GCLOUD_TESTS_GOLANG_PROJECT_ID -k $GCLOUD_TESTS_GOLANG_KEYRING/cryptoKeys/key1
+```
+
+It may be useful to add exports to your shell initialization for future use.
+For instance, in `.zshrc`:
+
+```sh
+#### START GO SDK Test Variables
+# Developers Console project's ID (e.g. bamboo-shift-455) for the general project.
+export GCLOUD_TESTS_GOLANG_PROJECT_ID=your-project
+
+# The path to the JSON key file of the general project's service account.
+export GCLOUD_TESTS_GOLANG_KEY=~/directory/your-project-abcd1234.json
+
+# Developers Console project's ID (e.g. doorway-cliff-677) for the Firestore project.
+export GCLOUD_TESTS_GOLANG_FIRESTORE_PROJECT_ID=your-firestore-project
+
+# The path to the JSON key file of the Firestore project's service account.
+export GCLOUD_TESTS_GOLANG_FIRESTORE_KEY=~/directory/your-firestore-project-abcd1234.json
+
+# The full name of the keyring for the tests, in the form "projects/P/locations/L/keyRings/R".
+# The creation of this is described below.
+export MY_KEYRING=my-golang-sdk-test
+export MY_LOCATION=global
+export GCLOUD_TESTS_GOLANG_KEYRING=projects/$GCLOUD_TESTS_GOLANG_PROJECT_ID/locations/$MY_LOCATION/keyRings/$MY_KEYRING
+
+# API key for using the Translate API.
+export GCLOUD_TESTS_API_KEY=abcdefghijk123456789
+
+# Compute Engine zone. (https://cloud.google.com/compute/docs/regions-zones)
+export GCLOUD_TESTS_GOLANG_ZONE=your-chosen-region
+#### END GO SDK Test Variables
 ```
 
 #### Running
@@ -187,7 +221,15 @@ Once you've done the necessary setup, you can run the integration tests by
 running:
 
 ``` sh
-$ go test -v cloud.google.com/go/...
+$ go test -v ./...
+```
+
+Note that the above command will not run the tests in other modules. To run
+tests on other modules, first navigate to the appropriate
+subdirectory. For instance, to run only the tests for datastore:
+``` sh
+$ cd datastore
+$ go test -v ./...
 ```
 
 #### Replay