Add ARCHITECTURE file

ARCHITECTURE.md

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+# Architecture
+
+  1. Arguments are passed to an instance of RedditDownloader
+  2. Internal objects are created
+     
+      - Formatter created
+      - Filter created
+      - Configuration loaded
+      - Reddit instance created
+     
+  3. Reddit lists scraped
+  
+To actually download, the following happens:
+  
+  1. RedditDownloader uses DownloadFactory to find the right module for a submission
+  2. Downloader instance created
+  3. Downloader returns a list of Resource objects (lists may have one objects)
+  4. RedditDownloader checks if it already exists
+  5. RedditDownloader checks against the DownloadFilter created earlier
+  6. RedditDownloader creates a formatted file path base on the Resource with FileNameFormatter
+  7. Resource content is written to disk
+