Remove non-license explanatory text from LICENSE.txt.
Create CONTRIBUTING file with instructions and notes for contributors.
This change relates to issue #412
--- /dev/null
+CONTRIBUTIONS and CODE REVIEWS
+
+We love contributions from the community. Please send pull requests and raise
+issues! All code submissions require review. We use GitHub pull requests for
+this purpose.
+
+
+COPYRIGHTS and LICENSE
+
+nMigen is licensed under the 2-clause BSD license, which is contained in the
+LICENSE.txt file.
+
+All authors retain copyright ownership of their contributions.
+
+Authors should note that all contributions are licensed under the 2-clause BSD
+license.
+
+
+CONTRIBUTORS
+
+The complete list of contributors is contained within the git repository.
+Tools such as the git log command can be used to see all contributions and
+contributors.
+Copyright (C) 2019-2020 whitequark
Copyright (C) 2011-2019 M-Labs Limited
Redistribution and use in source and binary forms, with or without modification,