Document build from release archive and fix ToC link

Update the documentation to include minimal instructions for building
the velero binary from the GitHub Release "Source code" archive.

Correct the Download link in the Table of Contents for
build-from-scratch.md.

Signed-off-by: Darren Hart <dvhart@vmware.com>

docs/build-from-scratch.md

@@ -1,7 +1,7 @@
 # Build from source
 
 * [Prerequisites][1]
-* [Download][2]
+* [Getting the source][2]
 * [Build][3]
 * [Test][12]
 * [Run][7]
@@ -16,6 +16,8 @@
 
 ## Getting the source
 
+### Option 1) Get latest (recommended)
+
 ```bash
 mkdir $HOME/go
 export GOPATH=$HOME/go
@@ -26,6 +28,8 @@ Where `go` is your [import path][4] for Go.
 
 For Go development, it is recommended to add the Go import path (`$HOME/go` in this example) to your path.
 
+### Option 2) Release archive
+Download the archive named `Source code` from the [release page][22] and extract it in your Go import path as `src/github.com/heptio/velero`.
 
 ## Build
 
@@ -41,6 +45,11 @@ make container
 
 To push your image to a registry, use `make push`.
 
+To build only the `velero` binary, run:
+```
+go build ./cmd/velero
+```
+
 ### Update generated files
 
 The following files are automatically generated from the source code:
@@ -204,7 +213,7 @@ If you need to add or update the vendored dependencies, see [Vendoring dependenc
 
 [0]: ../README.md
 [1]: #prerequisites
-[2]: #download
+[2]: #getting-the-source
 [3]: #build
 [4]: https://blog.golang.org/organizing-go-code
 [5]: https://golang.org/doc/install
@@ -223,3 +232,4 @@ If you need to add or update the vendored dependencies, see [Vendoring dependenc
 [19]: ../examples/README.md
 [20]: api-types/backupstoragelocation.md
 [21]: api-types/volumesnapshotlocation.md
+[22]: https://github.com/heptio/velero/releases