add README

README.md

@@ -1,3 +1,27 @@
 # Hex Editor for fx-CG
 
 This is a hex editor for fx-CG. Builds with [gint](https://gitea.planet-casio.com/Lephenixnoir/gint) and [JustUI](https://gitea.planet-casio.com/Lephenixnoir/JustUI).
+
+![](screenshots/hexedit.gif)
+
+The main features are:
+
+* Open and files loaded to RAM (up to ~300 kiB) with both overwrite and insertion mode
+* Load large files lazily and edit them in overwrite mode while partially loaded
+* View and edit the calculator's main memory areas
+* Create and save new files from scratch
+* Basic navigation and statistics tools
+
+**Data sources and the front buffer**
+
+The data source is the file or memory being edited. Accesses to the data source are cached through the *front buffer*, which is required both for functionality (we're not going to do a flash write for every change in a lazily-loaded file) and for performance (inserting in a 300-kiB file is not ideal).
+
+The status bar shows `Buffer: @POSITION SIZE/CAPACITY`, which tells where the front buffer is in the file, how much data from the source is loaded in the buffer, and what the capacity of the buffer is.
+
+When the front buffer has been modified but not yet saved to the data source, the title bar has a double star `**` next to the source name. When the source itself has been modified but not saved, it shows a single star `*`. This only happens for loaded files, since for lazy files and memory the front buffer saves directly into the data source.
+
+**About editing G3A files**
+
+Writing to G3A files from within an add-in is not supported by the OS, as it add-in registration, which destroys any running add-in's MMU mappings. [See this Cemetech thread](https://www.cemetech.net/forum/viewtopic.php?p=299466).
+
+If you write to a G3A file from the hex editor, the editor is likely to System ERROR or crash after writing. However, that's still fairly safe, so if you really need to edit a G3A file this way you can do it. Prefer using the loaded file mode, as the lazy file mode saves automatically when you move around after editing. If a System ERROR or crash occurs after saving, or if the add-ins don't show up in the main menu after leaving, just RESET the calc. The writes to G3A files do work, it's just that the add-in can't keep running after they're done.