all: add local caching

This commit adds config params to enable local DNS response caching and
control its behavior, allow tweaking the cache size, ttl override and
serving stale response.

docs/config.md

@@ -33,6 +33,8 @@ If no configuration files found, a default `config.toml` file will be created in
 [service]
     log_level = "info"
     log_path = ""
+    cache_enable = true
+    cache_size = 4096
 
 [network.0]
     cidrs = ["0.0.0.0/0"]
@@ -109,6 +111,31 @@ Relative or absolute path of the log file.
 - Type: string
 - Required: no
 
+### cache_enable
+When `cache_enable = true`, all resolved DNS query responses will be cached for duration of the upstream record TTLs.
+
+- Type: boolean
+- Required: no
+
+### cache_size
+The number of cached records, must be a positive integer. Tweaking this value with care depends on your available RAM. 
+A minimum value `4096` should be enough for most use cases.
+
+An invalid `cache_size` value will disable the cache, regardless of `cache_enable` value.
+
+- Type: int
+- Required: no
+
+### cache_ttl_override
+When `cache_ttl_override` is set to a positive value (in seconds), TTLs are overridden to this value and cached for this long.
+
+- Type: int
+- Required: no
+
+### cache_serve_stale
+When `cache_serve_stale = true`, in cases of upstream failures (upstreams not reachable), `ctrld` will keep serving
+stale cached records (regardless of their TTLs) until upstream comes online.
+
 The above config will look like this at query time.
 
 ```