doc: Add initial version of lib-store documentation/definition

doc/src/04000-lib-store.md

@@ -4,3 +4,67 @@
     Store functionality
 -->
 
+The "libstore" MUST define the programming-language level interface to the
+store on the file system.
+The library therefor MUST define and export types which can be used to get data
+from the filesystem.
+
+## Types {#sec:libstore:types}
+
+The types in @tbl:libstore:types are exported by the library.
+
+| Type         | Meaning                                          |
+| :----------- | :----------------------------------------------- |
+| Entry        | Entity on the Filesystem, File                   |
+| EntryContent | User-Content of the Entry                        |
+| EntryHeader  | Header of the Entry                              |
+| Store        | Store interface                                  |
+
+Table: Types the store library exports {#tbl:libstore:types}
+
+Each of these types MUST export functions to work with the data the objects of
+the types contain.
+
+### Entry {#sec:libstore:types:entry}
+
+The `Entry` type MUST hold the following content:
+
+- A path where on the filesystem the acutal file lives
+- An instance of `EntryContent` as interface to the content of the file
+  (@sec:libstore:types:entrycontent).
+- An instance of `EntryHeader` as interface to the header of the file
+  (@sec:libstore:types:entryheader).
+
+The entry type MUST export functions to get
+
+- The content object
+- The header object
+- The path of the actual file
+
+### EntryContent {#sec:libstore:types:entrycontent}
+
+The `EntryContent` type is an type-alias for `String`.
+
+### EntryHeader {#sec:libstore:types:entryheader}
+
+The `EntryHeader` type is an wrapper around the type, the TOML-Parser library
+exports.
+
+It SHOULD contain utility functions to work with the header in a convenient way.
+
+### Store {#sec:libstore:types:store}
+
+The `Store` type MUST represent the interface to the store on the filesystem.
+It MUST contain CRUD-functionality to work with the entries in the store.
+It MUST contain a variable which contains the path of the store on the
+filesystem which is represented by an object of this type.
+It also MUST contain a getter for this variable.
+It MUST NOT contain a setter for this variable, as changing the store while the
+programm is running is not allowed.
+
+The `Store` object MAY contain caching functionality.
+If the `Store` type contains caching functionality for filesystem operations,
+the interface MUST NOT differ from the non-caching interface.
+If the `Store` type contains caching functionality, these functionality MUST NOT
+BE visible to the user of the library.
+