update testing readme

prevents `fatal: no submodule mapping found in .gitmodules for path 'test/test_helper/bats-core'`
so-fancy · Mar 4, 2016 · d166cf1 · d166cf1
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diff --git a/readme.md b/readme.md
@@ -109,6 +109,7 @@ cd ~/projects/catfabulator && git diff
 You'll need to install [bats](https://github.com/sstephenson/bats#installing-bats-from-source), the Bash automated testing system. It's also available as `brew install bats`
 
 ```sh
+git submodule sync
 git submodule update --init  # pull in the assertion library, bats-assert
 
 # Run the test suite once: