add commands doc

doc/commands.md

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+## Total
+
+ledisdb use redis protocol called RESP(REdis Serialization Protocol), [here](http://redis.io/topics/protocol).
+
+ledisdb all commands return RESP fomrat. Later I will use int64 refer RESP integer, string refer RESP simple string, bulk string refer RESP bulk string, and array refer RESP arrays.
+
+## KV 
+
+### decr key
+Decrements the number stored at key by one. If the key does not exist, it is set to 0 before decrementing.
+An error returns if the value for the key is a wrong type that can not be represented as a signed 64 bit integer.
+
+**Return value**
+
+int64: the value of key after the decrement
+
+**Examples**
+
+```
+ledis> decr mykey
+(integer) -1
+ledis> decr mykey
+(integer) -2
+ledis> SET mykey "234293482390480948029348230948"
+OK
+ledis> decr mykey
+ERR strconv.ParseInt: parsing "234293482390480948029348230948“: invalid syntax
+```
+
+### decrby key decrement
+
+Decrements the number stored at key by decrement. like decr.
+
+**Return value**
+
+int64: the value of key after the decrement
+
+**Examples**
+
+```
+ledis> set mykey “10“
+OK
+ledis> decrby mykey “5“
+(integer) 5
+```
+
+### del
+
+Removes the specified keys.
+
+**Return value**
+
+int64: The number of input keys 
+
+**Examples**
+
+```
+ledis> set key1 "hello"
+OK
+ledis> set key2 "world"
+OK
+ledis> del key1 key2
+(integer) 2
+```
+
+### exists
+
+Returns if key exists
+
+**Return value**
+
+int64, specifically:
+- 1 if the key exists.
+- 0 if the key does not exists.
+
+**Examples**
+
+```
+ledis> set key1 "hello"
+OK
+ledis> exists key1
+(integer) 1
+ledis> exists key2
+(integer) 0
+```
+
+### get
+
+Get the value of key. If the key does not exists, it returns nil value.
+
+**Return value**
+
+bulk: the value of key, or nil when key does not exist.
+
+
+**Examples**
+
+```
+
+```
+
+### getset
+**Return value**
+
+**Examples**
+
+### incr
+**Return value**
+
+**Examples**
+
+### incrby
+**Return value**
+
+**Examples**
+### mget
+**Return value**
+
+**Examples**
+### mset
+**Return value**
+
+**Examples**
+### set
+**Return value**
+
+**Examples**
+### setnx
+**Return value**
+
+**Examples**
+### expire
+**Return value**
+
+**Examples**
+### expireat
+**Return value**
+
+**Examples**
+### ttl
+**Return value**
+
+**Examples**
+### persist
+**Return value**
+
+**Examples**
+
+## Hash
+
+### hdel
+**Return value**
+
+**Examples**
+### hexists
+**Return value**
+
+**Examples**
+### hget
+**Return value**
+
+**Examples**
+### hgetall
+**Return value**
+
+**Examples**
+### hincrby
+**Return value**
+
+**Examples**
+### hkeys
+**Return value**
+
+**Examples**
+### hlen
+**Return value**
+
+**Examples**
+### hmget
+**Return value**
+
+**Examples**
+### hmset
+**Return value**
+
+**Examples**
+### hset
+**Return value**
+
+**Examples**
+### hvals
+**Return value**
+
+**Examples**
+
+## List
+
+### lindex
+**Return value**
+
+**Examples**
+### llen
+**Return value**
+
+**Examples**
+### lpop
+**Return value**
+
+**Examples**
+### lrange
+**Return value**
+
+**Examples**
+### lpush
+**Return value**
+
+**Examples**
+### rpop
+**Return value**
+
+**Examples**
+### rpush
+**Return value**
+
+**Examples**
+### lclear
+**Return value**
+
+**Examples**
+### lexpire
+**Return value**
+
+**Examples**
+### lexpireat
+**Return value**
+
+**Examples**
+### lttl
+**Return value**
+
+**Examples**
+### lpersist 
+**Return value**
+
+**Examples**
+
+## ZSet
+
+### zadd
+**Return value**
+
+**Examples**
+### zcard
+**Return value**
+
+**Examples**
+### zcount
+**Return value**
+
+**Examples**
+### zincrby
+**Return value**
+
+**Examples**
+### zrange
+**Return value**
+
+**Examples**
+### zrangebyscore
+**Return value**
+
+**Examples**
+### zrank
+**Return value**
+
+**Examples**
+### zrem
+**Return value**
+
+**Examples**
+### zremrangebyrank
+**Return value**
+
+**Examples**
+### zremrangebyscore
+**Return value**
+
+**Examples**
+### zrevrange
+**Return value**
+
+**Examples**
+### zrevrangebyscore
+**Return value**
+
+**Examples**
+### zscore
+**Return value**
+
+**Examples**
+### zclear
+**Return value**
+
+**Examples**
+### zexpire
+**Return value**
+
+**Examples**
+### zexpireat
+**Return value**
+
+**Examples**
+### zttl
+**Return value**
+
+**Examples**
+### zpersist
+**Return value**
+
+**Examples**
+
+## Replication
+
+### slaveof
+**Return value**
+
+**Examples**
+### fullsync
+**Return value**
+
+**Examples**
+### sync
+**Return value**
+
+**Examples**
+
+## Server
+
+### ping
+**Return value**
+
+**Examples**
+### echo
+**Return value**
+
+**Examples**
+### select
+**Return value**
+
+**Examples**