Add public profile to GitHub organisation (#20)

Taken from the website homepage. See @UCL-MIRSG for an example.

profile/README.md

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+# UCL Centre for Advanced Research Computing
+
+> ARC is UCL's research, innovation and service centre for the tools, practices
+> and systems that enable computational science and digital scholarship.
+
+We are an innovative centre with both professional services and academic
+missions:
+
+- We provide advanced, reliable and secure digital research infrastructure to
+  research groups in UCL and beyond.
+- We are a laboratory for research, teaching and innovation in compute, data and
+  software intensive research methods.
+
+For more details, see our [website](https://www.ucl.ac.uk/arc).