Inclusion of Bazel build files in official zstd library

**Is your feature request related to a problem? Please describe.**
I need to use this library to build RocksDB. We use bazel to build everything, which means zstd must build with bazel.

Currently we are achieving bazel builds by pulling in https://github.com/cschuet/zstd/pull/2, which tends to go stale.

**Describe the solution you'd like**
Add a BUILD.bazel and WORKSPACE file to the zstd repo. Bazel users can use this and help maintain it.

**Describe alternatives you've considered**
1. Out-of-repository bazel projects like https://github.com/cschuet/zstd/pull/2
2. Using CMake (or whatever zstd uses) within bazel.

I know only a casual amount about the C++ build ecosystem, so I'm not sure what all the options are. Bazel is nice because users can issue a uniform set of commands across all languages, so I don't /have/ to know the C++ ecosystem to build C++ tools

**Additional context**
Would the authors accept a pull request with BUILD.bazel and WORKSPACE files?

See https://github.com/cschuet/zstd/blob/02cedda8da27c2043166dec046599e13da4a68b6/bazel/third_party/zstd.BUILD for expected contents.