Update slack link (#178)

* Updating slack link

Author: Keith Roberts

README.md

@@ -26,7 +26,9 @@ OceanMesh2D is a set of user-friendly MATLAB functions to generate two-dimension
 Getting help
 ==============
 
-Besides posting [issues](https://github.com/CHLNDDEV/OceanMesh2D/issues) with the code on Github, you can also ask questions via our Slack channel [here](https://join.slack.com/t/oceanmesh2d/shared_invite/zt-igvr8rvo-OctG9t0QEYNyyvVr55ynUQ).
+Besides posting [issues](https://github.com/CHLNDDEV/OceanMesh2D/issues) with the code on Github, you can also ask questions via our Slack channel [here](https://join.slack.com/t/oceanmesh2d/shared_invite/zt-ldexcc7m-LkkxsUTUekHbJNxwlyrMYQ).
+
+Note: If the slack link invite isn't working, please send either one of us and an email and we'll fix it. By default, the invite link expires every 30 days. 
 
 Otherwise please reach out to either Dr. William Pringle ([email protected]) or Dr. Keith Roberts ([email protected]) with questions or concerns or feel free to start an Issue in the issues tab above.