Add a Gitter chat badge to README.md (#10)

Author: gitter-badger

README.md

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 # rustboot
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+[![Join the chat at https://gitter.im/rust-osdev/bootloader](https://badges.gitter.im/rust-osdev/bootloader.svg)](https://gitter.im/rust-osdev/bootloader?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
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 An experimental pure-Rust x86 bootloader for the [planned second edition](https://github.com/phil-opp/blog_os/issues/360) of the [Writing an OS in Rust](https://os.phil-opp.com) series.
 
 **This is still work in progress**.