Write a big fat disclaimer README.md #3

1. 
   sipa commented at 2:20 pm on March 26, 2014: contributor

   This library is experimental, use at your own risk, …
2. 
   sipa commented at 11:06 pm on June 15, 2014: contributor

   Done.
3. sipa closed this on Jun 15, 2014

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4. benma referenced this in commit 7abe39b298 on Jul 15, 2019
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