Updated URLs for citations.

README.rst

@@ -17,8 +17,8 @@ Using gprMax? Cite us.
 
 If you use gprMax and publish your work we would be grateful if you could cite gprMax using the following references:
 
-* Warren, C., Giannopoulos, A., & Giannakis I. (2015). An advanced GPR modelling framework – the next generation of gprMax, In `Proc. 8th Int. Workshop Advanced Ground Penetrating Radar`
-* Giannopoulos, A. (2005). Modelling ground penetrating radar by GprMax, `Construction and Building Materials`, 19(10), 755-762. doi:10.1016/j.conbuildmat.2005.06.007
+* Warren, C., Giannopoulos, A., & Giannakis I. (2015). An advanced GPR modelling framework – the next generation of gprMax, In `Proc. 8th Int. Workshop Advanced Ground Penetrating Radar` (http://dx.doi.org/10.1109/IWAGPR.2015.7292621)
+* Giannopoulos, A. (2005). Modelling ground penetrating radar by GprMax, `Construction and Building Materials`, 19(10), 755-762 (http://dx.doi.org/10.1016/j.conbuildmat.2005.06.007)
 
 Software structure
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