Added some data sources

data/sources.md

+# Data sources
+
+## Global plate motions
+
+## Earthquake data
+
+- [USGS earthquake catalog](https://earthquake.usgs.gov/earthquakes/search/)
+- [Global CMT catalog](http://www.globalcmt.org/CMTsearch.html)
+- [IRIS data sources page](http://ds.iris.edu/sm2/html/SM_sources_current.html)
+
+## Volcano data
+
+- [Smithsonian global volcano program](https://volcano.si.edu/)
\ No newline at end of file