Tweak the README.md with respect to the repo subdirectories.

Change-Id: Ic883e7bebfaa20b6f62c5e734fed5e081031d2fc
Reviewed-on: https://weave-review.googlesource.com/3100
Reviewed-by: John Mccullough <jmccullough@google.com>

README.md

diff --git a/README.md b/README.md
index 38059db..428b0f5 100644
--- a/README.md
+++ b/README.md
@@ -29,11 +29,15 @@
 # Checkout code
 
 ```
+mkdir ~/weave
+cd ~/weave
 repo init -u https://weave.googlesource.com/weave/manifest
 repo sync
 ```
 
-# Directory structure
+This checks out libweave and its dependencies into the ~/weave directory.
+
+# libweave Directory structure
 
 | Path                     | Description                        |
 |--------------------------|------------------------------------|
@@ -94,6 +98,8 @@
 
 # Compiling
 
+From the `libweave` directory:
+
 The `make --jobs/-j` flag is encouraged, to speed up build time. For example
 
 ```