Created Use Config File for Command Line Options (markdown)

Use-Config-File-for-Command-Line-Options.md

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+Direct command line can get too long:
+
+```
+weed -logdir=../mlog master -mdir=../mdir -peers=192.168.126.5:3333,192.168.126.16:3333,192.168.126.18:3333 -port=3333 -defaultReplication=001
+```
+
+You can put these command line options into a configuration file :
+
+```
+# master.conf
+logdir=../mlog
+mdir=../mdir
+peers=192.168.126.5:3333,192.168.126.16:3333,192.168.126.18:3333
+port=3333
+defaultReplication=001
+```
+
+then refer it as 
+```
+weed master -config=master.conf
+```
+
+Order of precedence of this flag, it is borrowed from https://github.com/namsral/flag with some customized for weed:
+```
+1. Command line options
+2. Environment variables
+3. Configuration file
+4. Default values
+```