put docs in the middleware directory

middleware/log/log.md

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+# Log
+
+log enables request logging. The request log is also known from some vernaculars as an access log.
+
+## Syntax
+
+~~~
+log
+~~~
+
+* With no arguments, an query log is written to query.log in the common log format for all requests
+    (base name = .).
+
+~~~
+log file
+~~~
+
+* file is the log file to create (or append to). The base path is assumed to be . .
+
+~~~
+log name file [format]
+~~~
+
+* `name` is the base name to match in order to be logged
+* `file` is the log file to create (or append to)
+* `format` is the log format to use (default is Common Log Format)
+
+## Log File
+
+The log file can be any filename. It could also be stdout or stderr to write the log to the console,
+or syslog to write to the system log (except on Windows). If the log file does not exist beforehand,
+CoreDNS will create it before appending to it.
+
+## Log Format
+
+You can specify a custom log format with any placeholder values. Log supports both request and response placeholders.
+
+## Log Rotation
+
+If you enable log rotation, log files will be automatically maintained when they get large or old.
+You can use rotation by opening a block on your first line, which can be any of the variations
+described above:
+
+~~~
+log ... {
+    rotate {
+    size maxsize
+    age  maxage
+    keep maxkeep
+    }
+}
+~~~
+
+* `maxsize` is the maximum size of a log file in megabytes (MB) before it gets rotated. Default is 100 MB.
+* `maxage` is the maximum age of a rotated log file in days, after which it will be deleted. Default is to never delete old files because of age.
+* `maxkeep` is the maximum number of rotated log files to keep. Default is to retain all old log files.
+
+## Examples
+
+Log all requests to a file:
+
+~~~
+log /var/log/query.log
+~~~
+
+Custom log format:
+
+~~~
+log . ../query.log "{proto} Request: {name} {type} {>id}"
+~~~
+
+With rotation:
+
+~~~
+log query.log {
+    rotate {
+        100 # Rotate after 100 MB
+        age  14  # Keep log files for 14 days
+        keep 10  # Keep at most 10 log files
+    }
+}