*: Add tidb flags document

tidb/configuration.md

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+# Configuration flags
+
+TiDB is configurable through command-line flags and environment variables.
+
+The official TiDB ports are 4000 for client requests and 10080 for status report.
+
+### --store
++ Storage type
++ Human-readable name for this member.
++ default: "goleveldb"
++ You can choose from memory, goleveldb, boltdb and tikv. The first three are all local storage. TiKV is a distributed storage.
+
+### --path
++ Path to the data directory for local storage (goleveldb, boltdb, memory). DSN to the distributed storage (TiKV).
++ default: "/tmp/tidb"
++ If use TiKV, the path should be in this format: $Host:$Port/pd?cluster=$ClusterID
+
+### -L
++ Log level
++ default: "debug"
++ You can choose from debug, info, warn, error, fatal.
+
+### -P
++ TiDB server listening port.
++ default: "4000"
++ TiDB server will accept MySQL client request from this port.
+
+### --status
++ TiDB server status report port.
++ default: "10080"
++ This is used for get server status.
+
+### --lease
++ Schema lease time in second.
++ default: "1"
++ It is schema lease time used in online schema change. Its value will affact DDL statement running time. It is very dangerous to change it if you don't understand the internal mechanism.
+
+### --socket
++ The socket file to use for connection.
++ default: ""
++ example: "/tmp/tidb.sock"
+
+### --perfschema
++ Enable(1) or disable(0) performance schema.
++ default: "0"
++ If set this to 1, it will affect performance.
+
+### TIDB_PPROF environment variable
++ Enable runtime profiling data via HTTP server. Address is at client URL + "/debug/pprof/"
++ If set $TIDB_PPROF to 0, TiDB will disable pprof. Otherwise TiDB will enable pprof.