Add README for 'tstypes' folder (#2103)

tstypes/README.md

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+## TypeScript definition files
+
+This module includes a [TypeScript](https://www.typescriptlang.org/)
+declaration file to enable auto complete in compatible editors and type
+information for TypeScript projects.
+
+### Credits
+
+The `*.d.ts` files in this folder are based on [DefinetlyTyped](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/54712a7e28090c5b1253b746d1878003c954f3ff/types/graphql) definitions written by:
+
+- TonyYang https://github.com/TonyPythoneer
+- Caleb Meredith https://github.com/calebmer
+- Dominic Watson https://github.com/intellix
+- Firede https://github.com/firede
+- Kepennar https://github.com/kepennar
+- Mikhail Novikov https://github.com/freiksenet
+- Ivan Goncharov https://github.com/IvanGoncharov
+- Hagai Cohen https://github.com/DxCx
+- Ricardo Portugal https://github.com/rportugal
+- Tim Griesser https://github.com/tgriesser
+- Dylan Stewart https://github.com/dyst5422
+- Alessio Dionisi https://github.com/adnsio
+- Divyendu Singh https://github.com/divyenduz
+- Brad Zacher https://github.com/bradzacher
+- Curtis Layne https://github.com/clayne11
+- Jonathan Cardoso https://github.com/JCMais
+- Pavel Lang https://github.com/langpavel
+- Mark Caudill https://github.com/mc0
+- Martijn Walraven https://github.com/martijnwalraven
+- Jed Mao https://github.com/jedmao