Add docs section for journal types

docs/journal-types.md

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+<!-- Copyright (C) 2012-2021 jrnl contributors
+     License: https://www.gnu.org/licenses/gpl-3.0.html -->
+# Journal Types
+`jrnl` can store your journal in a few different ways:
+
+ - a single text file (encrypted or otherwise)
+ - a folder structure organized by date containing unencrypted text files
+ - the DayOne Classic format, which is a folder structure containing 
+
+There is no need to specify what type of journal you'd like to use. Instead,
+`jrnl` will automatically detect the journal type based on whether you're
+referencing a file or a folder in your [config file](advanced.md),
+and if it's a folder, whether or not DayOne Classic content exists in it.
+
+
+
+## Single File
+The single file format is the most flexible, as it can be [encrypted](encryption.md).
+To use it, enter any path that is a file or does not already exist. You can
+use any extension. `jrnl` will automatically create the file when you save
+your first entry.
+
+## Folder
+The folder journal format organizes your entries into subfolders for the year
+and month and `.txt` files for each day. If there are multiple entries in a day,
+they all appear in the same `.txt` file.
+
+The directory tree structure is in this format: `YYYY/MM/DD.txt`. For instance, if
+you have an entry on May 5th, 2021 in a folder journal at `~/folderjournal`, it will
+be located in: `~/folderjournal/2021/05/05.txt`
+
+!!! note
+When creating a new folder journal, you will need to create the folder before running
+`jrnl`. Otherwise, when you run `jrnl` for the first time, it will assume that you
+are creating a single file journal instead, and it will create a file at that path.
+
+!!! note
+Folder journals can't be encrypted.
+
+## Day One Classic
+`jrnl` supports the original data format used by DayOne. It's very similar to the folder
+journal format, except it's identified by either of these characteristics:
+
+* the folder has a `.dayone` extension
+* the folder has a subfolder named `entries`
+
+This is not to be confused with the DayOne 2.0 format, which is very different.
+
+!!! note
+DayOne Classic journals can't be encrypted.