Add citation link to GMT6 paper and some other modules

Author: weiji14

README.rst

@@ -173,6 +173,10 @@ Developers". Feel free to cite our work in your research using the following Bib
 
 To cite a specific version of PyGMT, go to our Zenodo page at
 https://doi.org/10.5281/zenodo.3781524 and use the Export to BibTeX function there.
+It is also strongly recommended to cite the
+`GMT6 paper <https://doi.org/10.1029/2019GC008515>`__ (which PyGMT wraps around).
+Note that some modules like `surface` and `x2sys` also have their dedicated citation.
+Further information for all these can be found at https://www.generic-mapping-tools.org/cite.
 
 
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