add contribution notes

.github/CONTRIBUTING.md

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+Thank you for contributing flexboxgrid!
+=========================================
+
+## If you creating issue...
+
+Please write following issue template.
+Do not forget describing your platform and code example to reproduce.
+
+## If you creating pull request...
+
+Please write following pull request template.

.github/ISSUE_TEMPLATE.md

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+### Expected behavior
+
+Write here.
+
+### Actual behavior
+
+Write here.
+
+### Steps to reproduce behavior
+
+* OS:
+* Browser:
+* code/fiddle:

.github/PULL_REQUEST_TEMPLATE.md

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+### Description
+
+Write here
+
+### Check List
+
+- [ ] run the build script
+- [ ] bump the version number
+- [ ] update the html to reflect the new version number
+- [ ] Update the gh-pages branch