Add README.md file.

README.md

+ActiveTable XBlock
+==================
+
+This XBlock provides a tabular problem type, where students have to fill in some of the cells of a
+table.
+
+
+The table definition
+--------------------
+
+The table definition is entered in a Python-like syntax (actually in a strict subset of Python).  It
+must be a list of lists, with all inner lists having the same lengths.  The elements of the inner
+lists correspond to the cells of the table.  The first line contains the column headers and can only
+contain string literals.  All further lines represent the table body.  Cells can be either string
+literals, e.g. `'a string'`, numbers, e.g. `6.23`, or response cell declarations.  There are two
+types of response cells:
+
+    Numeric(answer=<correct_answer>, tolerance=<tolerance in percent>,
+            min_significant_digits=<number>, max_significant_digits=<number>)
+
+A cell that expects a numeric answer.  The tolerance is optional, and will default to the default
+tolerance specified above.  The restrictions for the number of significant digits are optional as
+well.  Significant digits are counted started from the first non-zero digit specified by the
+student, and include trailing zeros.
+
+    String(answer='<correct answer>')
+
+A cell that expects a string answer.
+
+An example of a table definition:
+
+    [
+        ['Event', 'Year'],
+        ['French Revolution', Numeric(answer=1789)],
+        ['Krakatoa volcano explosion', Numeric(answer=1883)],
+        ["Proof of Fermat's last theorem", Numeric(answer=1994)],
+    ]

activetable/activetable.py

@@ -36,12 +36,13 @@ class ActiveTableXBlock(StudioEditableXBlockMixin, XBlock):
     )
     help_text = String(
         display_name='Help text',
-        help='The text that gets displayed when clicking the "+help" button.  If you do not '
-        'specify a text, the help feature is disabled.',
+        help='The text that gets displayed when clicking the "+help" button.  If you remove the '
+        'help text, the help feature is disabled.',
         scope=Scope.content,
         multiline_editor=True,
         resettable_editor=False,
-        default='',
+        default='Fill in the cells highlighted in yellow with the correct answers.  '
+        'When you are done, you can check your answers using the button below the table.',
     )
     column_widths = String(
         display_name='Column widths',