WIP add info on file format

Author: Technius

docs/guide/file-format.md

@@ -0,0 +1,48 @@
+# File Format
+
+This page outlines the main file formats used by Xournal++. The primary file
+format is `.xopp`, with some plans to switch to more advanced file formats in
+the near future.
+
+## xopp
+
+The `.xopp` file format consists of a GZIP-compressed XML file. To inspect the
+XML contents, run a decompression command such as `gzip -d my_file.xopp` or
+`zcat my_file.xopp`. Structurally, an `.xopp` file is like an extended version
+of the `.xoj` file format used by the [Xournal program][xournal]; we briefly
+explain our extensions and changes in this section.
+
+(TODO: add text explaining the differences)
+
+## xopz
+
+!!! warning "Experimental Feature"
+
+    This is an experimental feature that is only available in unstable
+    development branches and is subject to change.
+
+The `.xopz` file format is the next generation file format to be used by
+Xournal++. A `.xopz` file is a zip file containing an "annotation" XML file that
+stores the main journal data, along with other resources such as PDF files,
+images, and audio. The XML file is the same as the one used in an `.xopp` file.
+This new file format is being developed to overcome limitations and shortcomings
+in the `.xopp` file format, including excessive file sizes and lack of support
+for more powerful features.
+
+The contents of the zip file are as follows:
+
+* `contents.xml`: the main annotation data stored in XML form; this is the same
+  format used in `.xopp`.
+* `META-INF/version`: FIXME: legacy code (not sure why this exists)
+* `background.pdf`: the background PDF used, if it exists.
+* `thumbnail.jpg`: a thumbnail of the document used for preview purposes.
+* `images/`: a folder containing images inserted into the document.
+* `latex/`: a folder containing rendered LaTeX images
+
+## xoj support in Xournal++
+
+Xournal++ supports limited export to the `.xoj` format supported by Xournal.
+However, there _will_ be limitations because the feature set of Xournal++ is
+much larger than that of Xournal.
+
+[xournal]: http://xournal.sourceforge.net/manual.html

mkdocs.yml

@@ -23,6 +23,7 @@ nav:
          - Notable plugins: guide/plugins/notable.md
       - File locations: guide/file-locations.md
       - Tips: guide/tips.md
+      - File Format: guide/file-format.md
   - Community: 
       - Getting help & helping out: community/help.md
       - Xournal++ & other software: community/other-software.md
@@ -63,5 +64,6 @@ markdown_extensions:
   - pymdownx.highlight
   - pymdownx.superfences
   - pymdownx.tabbed
+  - admonition
   - toc:
       permalink: true