add citation (#516)

Author: simonbyrne

README.md

@@ -22,3 +22,9 @@ For help and discussion, we suggest asking on the following venues:
 Contributions are encouraged. In particular, MPI provides several hundred functions, only a small number of which are currently exposed. If there are additional functions you would like to use, please open an [issue](https://github.com/JuliaParallel/MPI.jl/issues) or [pull request](https://github.com/JuliaParallel/MPI.jl/pulls).
 
 Additional examples and documentation improvements are also very welcome.
+
+# Citation
+
+If you use MPI.jl in your work, please cite the following paper:
+
+> Simon Byrne, Lucas Wilcox, and Valentin Churavy (2021) "MPI.jl: Julia bindings for the Message Passing Interface". _JuliaCon Proceedings_, 1(1), 68, doi: [10.21105/jcon.00068](https://doi.org/10.21105/jcon.00068)

docs/src/index.md

@@ -4,3 +4,7 @@ This is a basic [Julia](http://julialang.org/) wrapper for the portable message
 system Message Passing Interface ([MPI](http://www.mpi-forum.org/)). Inspiration is taken from
 [mpi4py](http://mpi4py.scipy.org), although we generally follow the C and not the C++ MPI API.
 (The C++ MPI API is deprecated.)
+
+If you use MPI.jl in your work, please cite the following paper:
+
+> Simon Byrne, Lucas Wilcox, and Valentin Churavy (2021) "MPI.jl: Julia bindings for the Message Passing Interface". _JuliaCon Proceedings_, 1(1), 68, doi: [10.21105/jcon.00068](https://doi.org/10.21105/jcon.00068)