Document PyOWM 2 stream maintenance timeline

[csparpa]

Lay out and publish a timeline for PyOWM 2 stream maintenance

PyOWM 2 stream should only be touched to fix bugs, no new features. The timeline should encompass an overall End-Of-Life date and should take into account the Python 2 LTS branch (v2.9-LTS) until its "natural" death on Jan 1, 2020

[csparpa]

Here is the timeline summarizing both Pyhon 2.x and PyOWM 2.x fate:

PyOWM Branches
^
|                    3.0
|                 O---------------------->>>  
|
|  2.10              bugfix only
O-----------------O--------------X
|
|   2.9-LTS
|  (python 2)
O----X
|
|
'----§------------§--------------§---------> Time
  Jan, 1st     PyOWM 3.0       +1 year
	2020      release date
	          (still TBC)

Can be also found in https://github.com/csparpa/pyowm/wiki/PyOWM-2-stream---Maintenance-plan