Add SH definitions for Germany 2018-2023

Closes: #192

Makefile

@@ -201,7 +201,7 @@ check-diff-%.js: %.js test.js
 	else \
 		echo "Test results for $< ($(CHECK_LANG)) produced a different output then the output of the current HEAD. Checkout the following diff."; \
 	fi
-	git --no-pager diff --exit-code -- test.$(CHECK_LANG).log
+	git --no-pager diff --exit-code -- "test.$(CHECK_LANG).log"
 
 .PHONY: osm-tag-data-taginfo-check
 osm-tag-data-taginfo-check: real_test.js opening_hours.js osm-tag-data-get-taginfo

holidays/README.md

@@ -160,6 +160,9 @@ SH:
 
 Note that the 4-digit keys define the year are in fact strings. This is done for compatibility reasons.
 
+Also note that past year definitions can be removed from the definition as long as the SH dataset can be regenerated as a whole by Free Software without depending on none-cached resources and as long as the unit tests pass which might where written against previous holidays.
+Two years in the past should be more then enough for the typical use cases of the library. In the far future, a compile time option might be provided to make this configurable to also make historian’s happy.
+
 ### Hints
 
 * Note that you should include the definitions in order (see [#126](https://github.com/opening-hours/opening_hours.js/issues/126#issuecomment-156853794) for details).