reduce tags in readme (#216)

Only 12 tags are evaluated, first 5 are displayed and combined tags are
redundant, so we reduce the number of tags presented, reorder them by
relevance and remove redundancies.

readme.txt

@@ -1,7 +1,7 @@
 # Cachify #
 * Contributors:      pluginkollektiv
 * Donate link:       https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=TD4AMD2D8EMZW
-* Tags:              acceleration, apc, cache, caching, compress, database cache, db-cache, disk cache, disk caching, google, hdd, html compression, memcached, minify, minimize, optimize, page cache, performance, quick cache, speed
+* Tags:              acceleration, cache, caching, minimize, performance, apc, disk, hdd, memcached, compression, minify, speed
 * Requires at least: 4.4
 * Tested up to:      5.7
 * Requires PHP:      5.2.4