Reference N817 from N814, and vice-versa

Also add pep8.org references for many of the rules.

README.rst

@@ -39,33 +39,43 @@ These error codes are emitted:
 | code | sample message                                        |
 +======+=======================================================+
 | N801 | class names should use CapWords convention            |
+|      | (https://pep8.org/#class-names)                       |
 +------+-------------------------------------------------------+
 | N802 | function name should be lowercase                     |
+|      | (https://pep8.org/#function-names)                    |
 +------+-------------------------------------------------------+
 | N803 | argument name should be lowercase                     |
+|      | (https://pep8.org/#function-and-method-arguments)     |
 +------+-------------------------------------------------------+
 | N804 | first argument of a classmethod should be named 'cls' |
+|      | (https://pep8.org/#function-and-method-arguments)     |
 +------+-------------------------------------------------------+
 | N805 | first argument of a method should be named 'self'     |
+|      | (https://pep8.org/#function-and-method-arguments)     |
 +------+-------------------------------------------------------+
 | N806 | variable in function should be lowercase              |
 +------+-------------------------------------------------------+
 | N807 | function name should not start and end with '__'      |
 +------+-------------------------------------------------------+
-+------+-------------------------------------------------------+
 | N811 | constant imported as non constant                     |
+|      | (https://pep8.org/#constants)                         |
 +------+-------------------------------------------------------+
-| N812 | lowercase imported as non lowercase                   |
+| N812 | lowercase imported as non-lowercase                   |
 +------+-------------------------------------------------------+
 | N813 | camelcase imported as lowercase                       |
 +------+-------------------------------------------------------+
 | N814 | camelcase imported as constant                        |
+|      | (distinct from ``N817`` for selective enforcement)    |
 +------+-------------------------------------------------------+
 | N815 | mixedCase variable in class scope                     |
+|      | (https://pep8.org/#constants,                         |
+|      | https://pep8.org/#method-names-and-instance-variables)|
 +------+-------------------------------------------------------+
 | N816 | mixedCase variable in global scope                    |
+|      | (https://pep8.org/#constants)                         |
 +------+-------------------------------------------------------+
 | N817 | camelcase imported as acronym                         |
+|      | (distinct from ``N814`` for selective enforcement)    |
 +------+-------------------------------------------------------+
 
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