documentation: add CI FAQ

documentation/ci_faq.md

+# Continuous Integration (CI) FAQ
+
+## Why did my commit fail CI?
+On every commit, a script runs that performs a number of things, including
+compiling code and running unit and functional tests. Whatever you did
+apparently broke one of these things.
+
+## Why should I care about the CI results?
+CI _immensely_ streamlines the development cycle by helping us catch bugs
+immediately, allowing us to fix them early before they plague us (or future
+users of the code base). It's a common industry practice. If you break the CI 
+pipeline, don't ignore it! Try your best to get the branch back in working
+condition.
+
+## Okay, so my commit failed CI, and I guess I care. What should I do?
+1. Re-run the build. 
+   - Sometimes something weird happens, so it's good to check this.
+2. Figure out what you did wrong and fix it. 
+   - If you click on the build, you should be able to see the exact logs the
+     build process. Debug the error to figure out what went wrong. Maybe you 
+     failed a test. You may need to open the source code for the test to see
+     where you failed, which is very insightful into figuring out what went
+     wrong.
+3. Figure out what CI is doing wrong.
+   - Sometimes a test might be poorly designed. At this point, it can be really
+     tempting to just ignore the build failure altogether (don't do that!), or
+     just remove the broken test (not as bad, but still don't do that). The best
+     thing is to fix the test so that we continue to have good regression
+     test coverage over our code.
+
+## How does CI work?
+When a commit is added to a branch in our repository, a notification is sent out
+to our gitlab-ci-runner, which instructs it to checkout the updated branch and
+run the directive in `.gitlab-ci.yml`. It then runs each directive, which is
+made up of bash scripts to execute. In our case, we run two scripts, 
+`ci-build.sh` and `ci-test.sh`, which compile projects and run checks,
+respectively. (Look at these scripts to learn more about how they work). If
+some error occurs during the script (perhaps a forced error generated by a
+failed test), then the runner sends a message back to Gitlab indicating that the
+CI failed. Otherwise, it succeeds.