Make theos a submodule

Author: davidjb

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "modules/Valet"]
 	path = modules/Valet
 	url = https://github.com/Square/Valet.git
+[submodule "modules/theos"]
+	path = modules/theos
+	url = https://github.com/rpetrich/theos.git

README.md

@@ -93,15 +93,15 @@ Building
 2. Build using the following:
 
    ```
-   mkdir pass-ios-build
-   git clone --recursive https://github.com/rpetrich/theos.git
    git clone --recursive https://github.com/davidjb/pass-ios.git
-   export THEOS=$(pwd)/theos
    cd pass-ios
+   source .env
    make
    make package
    ```
 
+   This clones and configures the theos environment for building.
+
 3. Install directly your device over SSH with:
 
    ```