pep-550: Add a section about context isolation (#331)

pep-0550.rst

@@ -968,6 +968,27 @@ trampoline, making it impossible to intercept their ``yield`` points
 outside of the Python interpreter.
 
 
+Is it possible to write a context manager to isolate EC changes?
+----------------------------------------------------------------
+
+Yes!
+
+::
+
+    @contextlib.contextmanager
+    def isolated_context():
+        old_ctx = sys.get_execution_context()
+        try:
+            yield
+        finally:
+            sys.set_execution_context(old_ctx)
+
+
+    with isolated_context():
+        # Any Execution Context changes will not be visible
+        # outside of this block.
+
+
 Reference Implementation
 ========================