CONTRIBUTING.md, doc: add details for local development

Instructions are added for how to install golang-migrate. Additional context is added for how to populate the local database, and run the frontend and worker servers.

Updates b/157994345

Change-Id: I937fd77da5894c01f2c380cc94055fa786a448ce
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/760281
CI-Result: Cloud Build <devtools-proctor-result-processor@system.gserviceaccount.com>
Reviewed-by: Julie Qiu <julieqiu@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

CONTRIBUTING.md

@@ -39,7 +39,11 @@ Before you begin working on an issue, please leave a comment that you are claimi
    wish to set up a similar environment, you will want to
    download and install the Google Cloud SDK at https://cloud.google.com/sdk/docs/.
 
-4. Depending on the feature you are working on, review the contributing guides for:
+4. If you are planning to develop any feature that involves the database, you will need
+   to [set up](doc/postgres.md) and [populate](doc/worker.md#populating-data-locally-using-the-worker) your local database. To run 
+  the frontend and server servers locally, see the documentation for the [frontend server](doc/frontend.md) and [worker server](doc/worker.md).
+
+5. Depending on the feature you are working on, review the contributing guides for:
 
 - [Frontend development](doc/frontend.md)
 - [Worker development](doc/worker.md)

doc/postgres.md

@@ -76,6 +76,10 @@ Migrations are managed using
 [github.com/golang-migrate/migrate](https://github.com/golang-migrate/migrate), with the
 [CLI tool](https://github.com/golang-migrate/migrate/tree/master/cli).
 
+If this is your first time using golang-migrate, check out the [Getting Started guide](https://github.com/golang-migrate/migrate/blob/master/GETTING_STARTED.md).
+
+To install the golang-migrate CLI, follow the instructions in the [migrate CLI README](https://github.com/golang-migrate/migrate/blob/master/cmd/migrate/README.md).
+
 ### Creating a migration
 
 To create a new migration:
@@ -104,4 +108,6 @@ Use the `migrate` CLI:
 devtools/migrate_db.sh [up|down|force|version] {#}
 ```
 
+If you are migrating for the first time, choose the "up" command.
+
 For additional details, see [golang-migrate/migrate/GETTING_STARTED.md#run-migrations](https://github.com/golang-migrate/migrate/blob/master/GETTING_STARTED.md#run-migrations).