[SUGGESTION] Get a place to put the to-do list, in-progress things of IoTDB

[weizihan0110]

Hi, all:
    Up to now, there's no place to find the to-do list, in-progress things of 
IoTDB. To make it more clear for contributors, I suggest we get a place to put 
these things. And most important is the place should be easily noticed by 
people. Below is an example from the questdb project:
https://github.com/questdb/questdb/projects/3