update: add basic setup guide with gotchas I ran into

Author: BrandonKlotz

.ruby-version

@@ -0,0 +1 @@
+3.1.6

setup.md

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+# Setup Guide
+
+1. Clone the repository
+2. `bundle install` dependencies
+3. `bundle exec rake db:create`
+4. `bin/rails db:migrate RAILS_ENV=test`
+
+rbenv local `3.1.6` (what Admin currently uses)
+
+Added getting the `bullet-train-billing` repo via GemFury in `Gemfile`
+```
+source "<Value from Admin Gemfile>@gem.fury.io/bullettrain" do
+  gem 'bullet_train-billing'
+end
+```
+
+Updated `charlock_holmes` to compatible version in `Gemfile.lock` (was unsure if I should commit this)
+```
+charlock_holmes (0.7.9)
+```
+
+To get all migrations to run successfully I had to update this migration's version from `7.1` -> `7.0`
+(was unsure if I should commit this)
+```
+class AddTrackableToBillingUsageTracker < ActiveRecord::Migration[7.0]
+  def change
+    add_column :billing_usage_trackers, :trackable_id, :bigint
+    add_column :billing_usage_trackers, :trackable_type, :string, default: "Team"
+    add_index :billing_usage_trackers, [:trackable_id, :trackable_type]
+  end
+end
+```