chore: update developer docs for testing (#60)

CONTRIBUTING.md

@@ -25,4 +25,39 @@ information on using pull requests.
 ## Community Guidelines
 
 This project follows
-[Google's Open Source Community Guidelines](https://opensource.google.com/conduct/).
+[Google's Open Source Community Guidelines](https://opensource.google.com/conduct/).
+
+
+## Running Tests
+
+Bigtable integration tests can either be run against an emulator or a real Bigtable instance.
+The target environment can be selected by setting a maven profile. By default it is set to 
+`bigtable-emulator-it` and other options are `bigtable-prod-it` and `bigtable-directpath-it`.
+
+To use the `bigtable-prod-it` and `bigtable-directpath-it` environments:
+
+1. Set up the target table using scripts/setup-test-table.sh
+2. Download the JSON service account credentials file from the Google Developer's Console.
+3. Set the environment variable GOOGLE_APPLICATION_CREDENTIALS to the path of the credentials file
+4. Enable the profile and the system properties `bigtable.project`, `bigtable.instance` 
+   and `bigtable.table` to created earlier. Example:
+  ```bash
+   mvn verify \
+  -P bigtable-prod-it \
+  -Dbigtable.project=my-project
+  -Dbigtable.instance=my-instance
+  -Dbigtable.table=my-table
+  ```
+
+While developing, it might be helpful to separate running unit tests from integration tests.
+
+```bash
+# To skip integration tests:
+mvn install -DskipITs
+
+# To skip unit tests
+mvn install -DskipUnitTests
+
+# To skip all tests
+mvn install -DskipTests
+```

google-cloud-bigtable/pom.xml

@@ -16,6 +16,11 @@
   </parent>
   <properties>
     <site.installationModule>google-cloud-bigtable</site.installationModule>
+
+    <!-- Enable the ability to skip unit tests and only run integration tests,
+         while still respecting global skipTests override. -->
+    <skipTests>false</skipTests>
+    <skipUnitTests>${skipTests}</skipUnitTests>
   </properties>
   <dependencies>
     <!-- NOTE: Dependencies are organized into two groups, production and test.
@@ -383,6 +388,8 @@
         </dependencies>
 
         <configuration>
+          <!-- enable the ability to skip unit tests, while running integration tests -->
+          <skipTests>${skipUnitTests}</skipTests>
           <!--
             TODO(igorbernstein): enable parallel tests once the generate client tests use unique
             names for the mock server