Update README.md

Added gslib refs.

README.md

@@ -107,6 +107,9 @@ libParanumal requires installed BLAS and LAPACK libraries. By default, the build
 Some Linux distributions will package BLAS and LAPACK libraries. For example, on Ubuntu systems these libraries can be installed via
 ```sudo apt install libblas-dev liblapack-dev```
 
+
+libParanumal also depends on the [gslib](https://github.com/Nek5000/gslib) library for gather-scatter operations. For more information on gslib see [Henry Tufo's thesis](https://dl.acm.org/doi/book/10.5555/926758) and a more recent reference [Fischer et al.](https://iopscience.iop.org/article/10.1088/1742-6596/125/1/012076/meta). The source code for gslib is included in this repository.
+
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 ### 7. Clone: libParanumal
 `git clone https://github.com/paranumal/libparanumal`