chore: Update README and switch to markdown format.

Details and link added for new repository location on GitHub.

readme.txt → README.md

-Description
-AirSeaFluxCode is a python 3.6 software designed to allow easy exploration of the different bulk formulae parameterizations and the wide range of implementation options.
+# `AirSeaFluxCode`
 
-Citation
+**This repository has now moved to GitHub.** Please see [https://github.com/NOCSurfaceProcesses/AirSeaFluxCode](https://github.com/NOCSurfaceProcesses/AirSeaFluxCode)
 
-Biri S, Cornes RC, Berry DI, Kent EC and Yelland MJ (2023). AirSeaFluxCode: Open source software for calculating turbulent air-sea fluxes from meteorological parameters. Front. Mar. Sci. 9:1049168. doi: 10.3389/fmars.2022.1049168
+## Description
+
+`AirSeaFluxCode` is a python 3.6 software designed to allow easy exploration of the different bulk formulae parameterizations and the wide range of implementation options.
+
+## Citation
+
+Biri S, Cornes RC, Berry DI, Kent EC and Yelland MJ (2023). AirSeaFluxCode: Open source software for calculating turbulent air-sea fluxes from meteorological parameters. Front. Mar. Sci. 9:1049168. doi: [10.3389/fmars.2022.1049168](https://www.frontiersin.org/articles/10.3389/fmars.2022.1049168/full)
 
 This work was supported by the Natural Environment Research Council grants ORCHESTRA, (NE/N018095/1), ENCORE (NE/V013254/1) and CLASS (NE/R015953/1).
 
-Licensing
+## Licensing
+
 Please see subdirectories for code and data licenses