add README (#2)

README.md

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+# bazel-github-actions-cache
+
+bazel-github-actions-cache is a Bazel Remote Caching server that uses Github Action's undocumented cache API as backend.
+
+Its goal is to utilize Actions Cache more efficiently, both in terms of speed and space, compared to [actions/cache](https://github.com/actions/cache).
+
+## Usage
+
+Add following step in your actions yaml file.
+```
+    - name: bazel-github-actions-cache
+      uses: tsawada/bazel-github-actions-cache@v0
+```
+
+Add following flag to Bazel.
+```
+--remote_cache=http://localhost:3055
+```
+
+## TODO
+
+* Proper error handling
+* Compression
+* gRPC interface