import lua purge extension to nginx

nginx-lua/purge/README.md

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+# Nginx proxy_cache invalidation
+
+The code here is presented as a two-article series on the scene-si.org blog:
+
+1. [Purging cached items from NGINX with LUA](https://scene-si.org/2016/11/02/purging-cached-items-from-nginx-with-lua/)
+2. Wildcard purge of cached items with NGINX and LUA (TODO)
+
+The development of the wildcard purge was sponsored by Carlos Mirando Molina, based on the
+discussion in the [reddit thread of the original post](https://www.reddit.com/r/nginx/comments/5apd4u/purging_cached_items_from_nginx_with_lua_tit/).
+A big thanks to him for supporting open source development.
+
+# Updates since the original version
+
+Both the purge-single.lua and purge-multi.lua now return a `X-Purged-Count` header, depending on how many files were
+found and deleted. For purge-single, the value is 0 if the item was not in the cache, and 1 if it was found. For
+purge-multi, the value will be the exact number of items found, which may be 0 to N (possibly several thousands).
+
+# Caveat emptor
+
+## Wildcard purge (purge-multi.lua)
+
+Purging items in such a manner requires `find`, `grep` and `xargs`. Depending on the amount of cached files that you have,
+and the hardware/VM resources you have available, a wildcard purge operation might be very expensive. With that in mind,
+you can consider many variants to optimize cache use.
+
+The complexity of the operation is `O(N+M)`, where N is the number of items in a cache, and M the number of items
+that must be purged from the cache. O(N) is done with an exec call (syscall, process), and O(M) is done with LUA code.
+
+With this in mind:
+
+1. Limit cache sizes, the more files you have, the slower it will be,
+2. Try to split up caches (many caches mean smaller purges),
+3. Wildcard purge is a heavy disk IO operation (pays to have SSD)
+
+## Single purge (purge-single.lua)
+
+Purging items in such a manner doesn't issue any syscalls apart from the deletion of the file. The complexity of this
+operation is `O(1)`. The request may have a minimal CPU impact, as a MD5 hash needs to be calculated.