README: Fix broken links.

Use import path of packages as the link name.

Author: dmitshur

README.md

@@ -17,9 +17,9 @@ Cache backends
 --------------
 
 - The built-in 'memory' cache stores responses in an in-memory map.
-- https://github.com/gregjones/httpcache/diskcache provides a filesystem-backed cache using the [diskv](https://github.com/peterbourgon/diskv) library.
-- https://github.com/gregjones/httpcache/memcache provides memcache implementations, for both App Engine and 'normal' memcache servers
-- https://github.com/sourcegraph/s3cache uses Amazon S3 for storage.
-- https://github.com/gregjones/httpcache/leveldbcache provides a filesystem-backed cache using [leveldb](https://github.com/syndtr/goleveldb/leveldb)
-- https://github.com/die-net/lrucache provides an in-memory cache that will evict least-recently used entries
-- 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/gregjones/httpcache/diskcache`](https://github.com/gregjones/httpcache/tree/master/diskcache) provides a filesystem-backed cache using the [diskv](https://github.com/peterbourgon/diskv) library.
+- [`github.com/gregjones/httpcache/memcache`](https://github.com/gregjones/httpcache/tree/master/memcache) provides memcache implementations, for both App Engine and 'normal' memcache servers.
+- [`sourcegraph.com/sourcegraph/s3cache`](https://sourcegraph.com/github.com/sourcegraph/s3cache) uses Amazon S3 for storage.
+- [`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.