Add codes and README doc for 212, 305, 306

Closes #9 and closes #10. Defines
 - `RST305`: Undefined substitution referenced: "XXX".
 - `RST306`: Unknown target name: "XXX".

Also defines a new 200-level code:
 - `RST212`: Title underline too short.

Updates README.rst to reflect these.

Author: bskinn

README.rst

@@ -99,6 +99,7 @@ RST207 Literal block ends without a blank line; unexpected unindent.
 RST208 Option list ends without a blank line; unexpected unindent.
 RST210 Inline strong start-string without end-string.
 RST211 Blank line required after table.
+RST212 Title underline too short.
 RST299 Previously unseen warning, not yet assigned a unique code.
 ====== =======================================================================
 
@@ -111,6 +112,8 @@ RST301 Unexpected indentation.
 RST302 Malformed table.
 RST303 Unknown directive type "XXX".
 RST304 Unknown interpreted text role "XXX".
+RST305 Undefined substitution referenced: "XXX".
+RST306 Unknown target name: "XXX".
 RST399 Previously unseen major error, not yet assigned a unique code.
 ====== =======================================================================
 

flake8_rst_docstrings.py

@@ -170,6 +170,7 @@ def tokenize_open(filename):
     # Other:
     "Inline strong start-string without end-string.": 10,
     "Blank line required after table.": 11,
+    "Title underline too short.": 12,
 }
 
 # Level 3 - error
@@ -180,6 +181,10 @@ def tokenize_open(filename):
     "Unknown directive type": 3,
     # e.g. Unknown interpreted text role "need".
     "Unknown interpreted text role": 4,
+    # e.g. Undefined substitution referenced: "dict".
+    "Undefined substitution referenced:": 5,
+    # e.g. Unknown target name: "license_txt".
+    "Unknown target name:": 6,
 }
 
 # Level 4 - severe