added the FAQ and a new folder for documentation

doc/faq.md

+
+**Table of content**
+
+[[_TOC_]]
+
+# How do I declare a LIST property?
+
+Use the datatype parameter (available with Property constructors and with the ```add_property``` method and the ```LIST``` function.
+
+```python
+#with constructor
+p = caosdb.Property(name="ListOfDoubles", datatype=caosdb.LIST(caosdb.DOUBLE))
+
+# with add_property method
+my_entity.add_property(name="ListOfIntegers", datatype=caosdb.LIST(caosdb.INTEGER))
+my_entity.add_property(name="ListOfThings", datatype=caosdb.LIST("Thing"))
+my_entity.add_property(name="ListOfThings", datatype=caosdb.LIST(caosdb.RecordType('Thing'))
+```
+
+# Which data types are there?
+
+There are 7 basic data types:
+
+* `INTEGER`
+* `DOUBLE`
+* `DATETIME`
+* `TEXT`
+* `BOOLEAN`
+* `FILE`
+* `REFERENCE`
+
+There is (so far) 1 data type for collections:
+
+* `LIST` (Well, LIST-of-another-data-type, e.g. `LIST(INTEGER)`)
+
+And furthermore,... 
+
+* Any RecordType can be used as a `REFERENCE` data type with a limited scope. That is, a property
+
+    ```python
+    p = caosdb.Property(name="Conductor", datatype="Person")
+    ```
+
+    will only accept those Entities as value which have a "Person" RecordType as a direct or indirect parent.
+
+See also: [Datatype](Datatype)