Add documentation around how specific triggers work

docs/Triggers.md

@@ -0,0 +1,29 @@
+# Triggers
+
+## Background
+
+The heart of any CI system is a set of triggers that kick off the builds when certain events happen, or on a schedule
+
+## Triggers and their events
+
+The events that we support at launch are:
+
+- `Nightly Trigger`: Build project builds on a schedule
+- `Commit Trigger`: Build after a commit is made
+- `PR Trigger`: Build after a PR is made, or a commit is added to a PR
+- `Manual Trigger`: Build after somebody presses the `Build` button
+
+For the initial release of fastlane.ci, our focus is:
+- Simple: Keeping things as simple as possible, and very easy to understand
+- Focused: Only worry about the primary scenarios of why people want to setup fastlane.ci
+- Flexible: developers can customize those further by modifying the JSON file manually. At least for a 1.0 that should be enough
+
+As a result, the `Add a new project step` has the following options:
+- Build every commit (for a specific branch, or all)
+- Build only PRs
+- Build nightly (at a given time)
+- Build manually only (for a specific branch)
+
+For the top 3 build triggers, we'd also automatically add the `manual trigger`. The primary reason is that we always want people to be able trigger a build manually. This is crucial especially during the setup, as most likely the first [x] builds won't succeed
+
+If the developer selects `nightly`, they get the extra selection for the time. All other trigger types don't have any additional options (beyond branch selection for some)