Updating build_scripts docs

Author: Tyler Mann

README.md

@@ -26,12 +26,21 @@ This plugin does not actually generate coverage data for you, since there are va
 (See more info at https://developer.apple.com/library/ios/qa/qa1514/_index.html)
 
 #####2. Setup the export_build_vars.py script in build phases
+
+This script is used to export any necessary environment variables from your latest build. The main reason this is needed is that many of the env variables at run-time are completely different in unrelated to your most recent build. They are also usually relative paths instead of absolute like these build variables.
+
 ![Export build vars](https://github.com/dropbox/XCoverage/raw/master/docs/export-build-vars.png)
 
 #####3. Add pre-action script to the test section of your scheme
+
+This script will clear any previous .gcda files from your target_object_dir, which helps ensure that this test run will be isolated in the coverage data.
+
 ![Pre-action](https://github.com/dropbox/XCoverage/raw/master/docs/pre-actions-setup.png)
 
 #####4. Add post-action script to the test section of your scheme
+
+This script will run gcov on all of your gcda files generated by your tests running in the app, generating .gcov files to be parsed by the plugin.
+
 ![Post-action](https://github.com/dropbox/XCoverage/raw/master/docs/post-actions-setup.png)