CONTRIBUTING.md: add a dependency update policy, and nightly vulncheck (#678)

Add a section the our CONTRIBUTING.md describing when and how
dependencies should be updated, and the criteria for adding a new
dependency.

Also, add a nightly workflow to run vulncheck.

Fixes #635

.github/workflows/nightly.yml

@@ -0,0 +1,17 @@
+name: Nightly
+
+on:
+  schedule:
+    - cron: '0 2 * * *'  # 2 AM UTC daily
+
+  workflow_dispatch:
+
+jobs:
+  vulncheck:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Run govulncheck
+      uses: golang/govulncheck-action@b625fbe08f3bccbe446d94fbf87fcc875a4f50ee # v1.0.4
+      with:
+           go-version-file: go.mod
+           go-package: ./...

CONTRIBUTING.md

@@ -105,6 +105,30 @@ copyright header following the format below:
 // license that can be found in the LICENSE file.
 ```
 
+### Adding and updating dependencies
+
+In general, the SDK tries to use as few dependencies as possible. Each new
+dependency is a potential source for bugs, churn, and conflicts for our users.
+Therefore, we require a [proposal](#proposals) for any new module dependency,
+including upgrading an existing module to a new major version. New dependencies
+should be evaluated for their stability and security, and should be
+well-established in the Go ecosystem.
+
+In general, dependencies should be for internal use by the SDK implementation,
+or for testing. Do not include types from dependencies in the SDK API.
+
+On the other hand, updating existing dependencies can be done at any time
+without a proposal, as long as their major version does not change. Prefer to
+update dependencies immediately following a release of the SDK, to allow as
+much time as possible to find issues with the new version.
+
+After any change to dependencies, run govulncheck to check them for
+vulnerabilities.
+
+```
+go run golang.org/x/vuln/cmd/govulncheck@latest
+```
+
 ### Updating the README
 
 The top-level `README.md` file is generated from `internal/readme/README.src.md`