Describe isort's multi_line_output setting (#109)

These changes follow up the "Fix and improve tests for Python != 3.7" PR.

The multi_line_output was already set to 3 in the
"Add initial black formatting" PR, so after rebasing to master
this commit contains only comment that describes a magic number
from the isort configuration file.

Corresponding PR:

 - open-telemetry/opentelemetry-python#109

Related discussions:

 - open-telemetry/opentelemetry-python#95 (comment)

 - open-telemetry/opentelemetry-python#95 (comment)

.isort.cfg

@@ -1,6 +1,14 @@
 [settings]
-multi_line_output=3
 include_trailing_comma=True
 force_grid_wrap=0
 use_parentheses=True
 line_length=79
+
+; 3 stands for Vertical Hanging Indent, e.g.
+; from third_party import (
+; lib1,
+; lib2,
+; lib3,
+; )
+; docs: https://github.com/timothycrosley/isort#multi-line-output-modes
+multi_line_output=3