[docs] auto year in copyright of ReadTheDocs (#1684)

docs/conf.py

@@ -17,6 +17,7 @@
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 #
+import datetime
 import os
 import sys
 import sphinx
@@ -71,7 +72,7 @@ master_doc = 'index'
 
 # General information about the project.
 project = 'LightGBM'
-copyright = '2017, Microsoft Corporation'
+copyright = '%s, Microsoft Corporation' % (str(datetime.datetime.now().year))
 author = 'Microsoft Corporation'
 
 # The version info for the project you're documenting, acts as replacement for