sweet: be explicit about how to get the repository

I always forget how to clone it to get started.

Change-Id: Ie3f04f8c99e73e89a56c70ef6a21750d43268b9b
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/670115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@google.com>

sweet/README.md

diff --git a/sweet/README.md b/sweet/README.md
index b4aeac4..4821b32 100644
--- a/sweet/README.md
+++ b/sweet/README.md
@@ -45,6 +45,13 @@
   `/proc/sys/kernel/yama/ptrace_scope` appropriately (0 and 1 work, 2 might,
   3 will not).
 
+### Download
+
+```sh
+$ git clone https://go.googlesource.com/benchmarks
+$ cd benchmarks/sweet
+```
+
 ### Build
 
 ```sh