Allow for external contributions to the FHE library

[cathieyun]

Currently, external contributions are not supported - potential contributors are advised to file issues in the project. In order to interface better with the open-source community and to increase the rate of development and improvement, we should support external contributions. This would require setting up an infrastructure similar to Tink (https://github.com/google/tink) for pulling external changes into the Google-internal codebase.