Update README.md

Minor edits to improve the README, added some important information from schema.rb comments.

Author: oddlittlebird

db/README.md

@@ -1,2 +1,7 @@
-Contains `modules_metadata_base.json` which contains information about all modules within Metasploit, as well as
-`schema.rb` which describes current state of the database schema maintained by Rails ActiveRecord.
+This directory contains the following files:
+
+- `modules_metadata_base.json`, which  information about all modules within Metasploit.
+- `schema.rb`, which is auto-generated from the current state of the database schema maintained by Rails ActiveRecord.
+  This file is auto-generated from the current state of the database.
+
+`schema.rb` is the source Rails uses to define your schema when running `bin/rails db:schema:load`. When creating a new database, `bin/rails db:schema:load` tends to be faster and is potentially less error prone than running all of your migrations from scratch. Old migrations may fail to apply correctly if those migrations use external dependencies or application code. We _strongly_ recommend that you check this file into your version control system.