Update submodule configuration for hub repo cloning efficiency (#75)

* Update submodules to not be auto cloned

* Update README about submoduling CMock

* Attempt to fix ci.yml

* Fix submodule init issue

* Add --recursive flags to submodule clone command

* Add section link in README

.github/workflows/ci.yml

@@ -13,6 +13,8 @@ jobs:
     steps:
       - name: Clone This Repo
         uses: actions/checkout@v2
+      - name: Checkout CMock submodule
+        run: git submodule update --checkout --init --recursive test/unit-test/CMock
       - name: Build
         run: |
           sudo apt-get install -y lcov

.gitmodules

@@ -1,9 +1,12 @@
 [submodule "test/unit-test/CMock"]
 	path = test/unit-test/CMock
 	url = https://github.com/ThrowTheSwitch/CMock
+	update = none
 [submodule "test/cbmc/aws-templates-for-cbmc-proofs"]
 	path = test/cbmc/aws-templates-for-cbmc-proofs
 	url = https://github.com/awslabs/aws-templates-for-cbmc-proofs.git
+	update = none
 [submodule "test/cbmc/litani"]
 	path = test/cbmc/litani
 	url = https://github.com/awslabs/aws-build-accumulator
+	update = none

README.md

@@ -29,6 +29,14 @@ For a CMake example of building the MQTT library with the `mqttFilePaths.cmake`
 
 ## Building Unit Tests
 
+### Checkout CMock Submodule
+By default, the submodules in this repository are configured with `update=none` in [.gitmodules](.gitmodules) to avoid increasing clone time and disk space usage of other repositories (like [amazon-freertos](https://github.com/aws/amazon-freertos) that submodule this repository.
+
+To build unit tests, the submodule dependency of CMock is required. Use the following command to clone the submodule:
+```
+git submodule update --checkout --init --recursive --test/unit-test/CMock
+```
+
 ### Platform Prerequisites
 
 - For running unit tests
@@ -39,7 +47,7 @@ For a CMake example of building the MQTT library with the `mqttFilePaths.cmake`
 
 ### Steps to build **Unit Tests**
 
-1. Go to the root directory of this repository.
+1. Go to the root directory of this repository. (Make sure that the **CMock** submodule is cloned as described [above](#checkout-cmock-submodule))
 
 1. Run the *cmake* command: `cmake -S test -B build -DBUILD_CLONE_SUBMODULES=ON `