Add a README.md file

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• Describe what's the purpose of this repository
• Add a note that the Linux binaries are experimental at this moment as we cannot (to my best knowledge) ensure that they will work on each and every distribution
• Describe (mainly for the LibGit2sharp contributors) the process of upgrading to a newest version of libgit2
  • Pointing to a new libgit2 version
  • What changes should be part of the PR in this repository
  • How to test that the produced NuGet package can be safely published (ie. Running the LibGit2sharp tests against it)

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Add a section explaining how to build the NuGet package (see #3 (comment) for inspiration, although that will surely be automated at some moment in time)

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Describe the process of upgrading from a fixed version to another one (see https://github.com/libgit2/libgit2sharp/pull/1049/files#r30443469)