add a readme

README.md

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+# YOPA
+
+Yopa is a relational database for cataloging things.
+
+The data model is user-editable through a web interface, together with the data.
+
+## Data model
+
+There are three kinds of entities in the database: objects, relations, and properties.
+
+Here is an example database schema that can be easily set up in Yopa:
+
+```
+      ,----quantity-----> Ingredient
+      |                   - amount in the pantry
+      |                   - e-shop link
+      |
+Recipe ------page X-----> Book
+- cook time               - author(s)
+- difficulty              - year
+- photo URI(s)
+```
+
+Relations link two objects. Properties are attached either to an object, or to a 
+relation - this makes sense for relations that need additional information, such as a 
+page number when referencing a book.
+
+Yopa implements four base data types: String, Integer, Decimal and Boolean. Properties 
+may further be optional and/or multiple.
+
+## Web interface
+
+Yopa is the database engine. There can be any number of user interfaces to it. Presently,
+the `yopa-web` module provides a web interface with a built-in web server. Just run the 
+binary and it will open your yopa database for editing. The file is created on the first
+change if it doesn't exist.
+  
+