Create per-rule pages and link from README (#2644)

docs/rules/assert-raises-exception.md

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+# assert-raises-exception (B017)
+
+### What it does
+Checks for the use of `assertRaises(Exception)`.
+
+### Why is this bad?
+`assertRaises(Exception)` can lead to your test passing even if the
+code being tested is never executed (e.g., due to a typo).
+
+Assert for a more specific exception (builtin or custom), use
+`assertRaisesRegex` or the context manager form of `assertRaises`.
+
+### Example
+```python
+self.assertRaises(Exception, foo)
+```
+
+Use instead:
+```python
+self.assertRaises(SomeSpecificException, foo)
+```