Merge branch 'develop' into publish-gh-pages

doc/index.md

@@ -17,6 +17,14 @@ the Wigner transport equation <LTC_options>`
 - API is prepared to operate phono3py from Python
   ([example](https://github.com/phonopy/phono3py/blob/master/example/Si-PBEsol/Si.py)).
 
+Papers that may introduce phono3py:
+
+- Theoretical background is summarized in this paper:
+  <http://dx.doi.org/10.1103/PhysRevB.91.094306> (arxiv
+  <http://arxiv.org/abs/1501.00691>).
+- Introduction to phono3py application:
+  <https://doi.org/10.1103/PhysRevB.97.224306> (open access).
+
 ```{image} Si-kaccum.png
 :width: 20%
 ```