SIM105: Add notice about speed impact in README

Closes #91

README.md

@@ -127,6 +127,25 @@ if a and b:
     c
 ```
 
+### SIM105
+
+```python
+# Bad
+try:
+    foo()
+except ValueError:
+    pass
+
+# Good
+from contextlib import suppress
+
+with suppress(ValueError):
+    foo()
+```
+
+Please note that `contextlib.suppress` is roughly 3x slower than `try/except`
+([source](https://github.com/MartinThoma/flake8-simplify/issues/91)).
+
 ### SIM107
 
 ```python