Define file format

doc/src/02000-store.md

@@ -2,5 +2,39 @@
 
 ## File Format {#sec:thestore:fileformat}
 
+The content in the store MUST BE encoded in either Unicode UTF-8 or ASCII.
+Each "Entry" (File) MUST HAVE a "Header" component as well as a "Content"
+component.
+Each "Entry" in the store MUST start with three single dashes ("-") followed
+by a newline character, named "initial marker" in the following chapter.
+The Header follows the initial marker (@sec:thestore:fileformat:header).
+The Header MUST BE followed by a line which contains three single dashes ("-")
+and a newline character, called "header-close marker" in the following
+chapter.
+The content follows the header-close marker (@sec:thestore:fileformat:content).
+
+### Header Format {#sec:thestore:fileformat:header}
+
+The header format MUST BE "TOML".
+The contents of the header contain
+
+1. A section called "imag", where the automatically by the program generated
+   data goes to.
+   The contents of these sections are edited via commandline calls or by the
+   program implicitely and SHOULD NOT be edited by the user.
+   The section MAY BE empty.
+1. Other OPTIONAL sections which are named and edited by the user. The program
+   MUST NOT touch the contents of these sections, except explicitely asked by
+   the user to do so.
+
+### Content Format {#sec:thestore:fileformat:content}
+
+The content is the part of the file where the user is free to enter any
+textual content.
+The content MAY BE rendered as Markdown or other markup format for the users
+convenience.
+The program SHOULD NOT expect any particular markup format, except explicitely
+configured in the header of the file.
+
 ## File organization {#sec:thestore:fileorganization}