Add the how to contribute doc to the rst file (#223)

Update the requirements.txt to include pre-commit

docs/index_cn.rst

@@ -5,4 +5,5 @@ VisualDL
     :maxdepth: 1
 
     quick_start_cn.md
+    contribute_code_cn.md
     api/visualdl.rst
\ No newline at end of file

docs/index_en.rst

@@ -6,4 +6,5 @@ VisualDL
 
     Home<README.md>
     quick_start_en.md
+    contribute_code_en.md
     api/visualdl.rst
\ No newline at end of file

requirements.txt

@@ -4,3 +4,4 @@ scipy==1.0.0
 Sphinx==1.6.6
 sphinx-rtd-theme==0.2.4
 flake8==3.5.0
+pre-commit==1.5.1