README: Add BoltDB implementation. (#78)

Author: gregjones

README.md

@@ -17,6 +17,7 @@ Cache Backends
 - [`github.com/gregjones/httpcache/leveldbcache`](https://github.com/gregjones/httpcache/tree/master/leveldbcache) provides a filesystem-backed cache using [leveldb](https://github.com/syndtr/goleveldb/leveldb).
 - [`github.com/die-net/lrucache`](https://github.com/die-net/lrucache) provides an in-memory cache that will evict least-recently used entries.
 - [`github.com/die-net/lrucache/twotier`](https://github.com/die-net/lrucache/tree/master/twotier) allows caches to be combined, for example to use lrucache above with a persistent disk-cache.
+- [`github.com/birkelund/boltdbcache`](https://github.com/birkelund/boltdbcache) provides a BoltDB implementation (based on the [bbolt](https://github.com/coreos/bbolt) fork).
 
 License
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