add test_filter argument to "make tests" command, example in README.rst

Makefile

@@ -6,7 +6,7 @@ docs:
 
 # run tests
 tests:
-	python -m unittest
+	python -m unittest ${test_filter}
 
 # check static typing
 check:

README.rst

@@ -62,10 +62,14 @@ Development
 
     make check
 
-* Run unit tests with::
+* Run all unit tests with::
 
     make tests
 
+* Run only some unit tests with::
+
+    make tests test_filter=tests.documents.test_block.TestBlock.test_fromraw
+
 Documentation
 -------------