Updated README with instructions to clean build cache.

[jonasclaes]

Due to a rare issue explained in #41, I've added instructions to clean the cache and try rebuilding.

[kentonv]

Hmm, I wonder if we should create a "build troubleshooting" document for tips like this? I suspect there will be move of these and it might clutter up the readme.

I think we've seen a few different types of errors caused by this, not just include-related, so I might describe it more like:

If you run a Bazel build before you've installed some dependencies (like clang or libc++), and then you install the dependencies, you must clean Bazel's cache, otherwise you might get strange errors.

[jonasclaes]

Hmm, I wonder if we should create a "build troubleshooting" document for tips like this? I suspect there will be move of these and it might clutter up the readme.

Whilst I'm not against the idea of a document containg troubleshooting "guides", at this moment I do not think it is necessary and we should cross this bridge when we get there?

I think we've seen a few different types of errors caused by this, not just include-related, so I might describe it more like:

If you run a Bazel build before you've installed some dependencies (like clang or libc++), and then you install the dependencies, you must clean Bazel's cache, otherwise you might get strange errors.

That's a great idea! I've never written the greatest documentation so I'm still learning as we go and this is a nice addition. Shall I update the PR?

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[jonasclaes]

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