Add CITATION.cff and update README

CITATION.cff

@@ -0,0 +1,27 @@
+# This CITATION.cff file was generated with cffinit.
+# Visit https://bit.ly/cffinit to generate yours today!
+
+cff-version: 1.2.0
+title: pysquirrel
+message: >-
+  If you use this package please cite the doi
+  corresponding to the release used
+type: software
+authors:
+  - given-names: David
+    family-names: Almeida
+    email: [email protected]
+    affiliation: >-
+      International Institute for Applied Systems Analysis (IIASA)
+    orcid: 'https://orcid.org/0009-0000-5458-7917'
+  - given-names: Daniel
+    family-names: Huppmann
+    affiliation: >-
+      International Institute for Applied Systems Analysis (IIASA)
+    orcid: 'https://orcid.org/0000-0002-7729-7389'
+  - given-names: Philip
+    family-names: Hackstock
+    affiliation: >-
+      International Institute for Applied Systems Analysis (IIASA)
+    orcid: 'https://orcid.org/0000-0002-1482-1366'
+license: MIT

README.md

@@ -38,15 +38,18 @@ parameters and the values to search as shown below:
 
 >>> pysquirrel.nuts.get(country_code="AT")  # gets all regions in Austria
 
->>> pysquirrel.nuts.get(code="PT191")  # gets the PT191 region
-
->>> pysquirrel.nuts.get(label="Drenthe")  # gets the Drenthe region
-
 >>> pysquirrel.nuts.get(level=3)  # gets all NUTS3 regions
 
->>> pysquirrel.nuts.get(parent_code="DE2")  # gets all regions whose parent region is DE2
+>>> pysquirrel.nuts.get(country_code="AT", level=3)  # gets all NUTS3 regions in Austria
 ```
 
+Each Region object consists of five attributes:
+- a NUTS code (e.g.: AT127)
+- a country code
+- a label (the full region name)
+- a NUTS level (1, 2 or 3)
+- a parent code (corresponding to the NUTS parent region)
+
 ## Eurostat copyright notice on NUTS region data file
 
 This package imports the NUTS spreadsheet from the Eurostat website.