Adds make-release script, static k8s manifests (#132)

Closes #75.  

### Changelog 

Adds 4 scripts to the `hack/` directory for building/pushing images, injecting images tags into static manifests, and tagging new releases. See [hack/README.md](https://github.com/m-okeefe/microservices-demo/tree/release-script/hack). 

**Note**: since we have not pushed images yet, the images in the `./release/` manifests are still set to the skaffold defaults (eg. `adservice`).

hack/README.md

@@ -0,0 +1,18 @@
+## `hack/` 
+
+This directory provides scripts for building and pushing Docker images, and tagging new demo
+releases. 
+
+### env variables 
+
+- `TAG` - git release tag / Docker tag. 
+- `REPO_PREFIX` - Docker repo prefix to push images. Format: `$user/$project`.  Resulting images will be of the
+  format `$user/$project/$svcname:$tag` (where `svcname` = `adservice`, `cartservice`,
+  etc.)
+
+### scripts 
+
+1. `./make-docker-images.sh`: builds and pushes images to the specified Docker repository.
+2. `./make-release-artifacts.sh`: generates a combined YAML file with image $TAG at: 
+   `./release/kubernetes-manifests/demo.yaml`. 
+3. `./make-release.sh`: runs scripts 1 and 2, then runs `git tag` / pushes updated manifests to master.