Add CNN masking install instruction

[tashrifbillah]

luigi-pnlpipe urges installation of pnlNipype. On the other hand, luigi-pnlpipe needs CNN masking tool. So--

• CNN masking should be cloned (in both pnlpipe and pnlNipype)
• CNN masking dependency should be included in requirements.txt: probably better to ask users to install pnlpipe hosted environment36.yml and omit pnlNipype hosted requirements.txt entirely. If we follow this route, environment36.yml should be wget from there to here. Another advantage of following this route is that we have tested environment36.yml well.

[tashrifbillah]

• Fixed by https://github.com/pnlbwh/luigi-pnlpipe/commit/0b2941a93d48a9aef84fd80ddaa92cc8180a9c1f We are cloning CNN masking in luigi-pnlpipe only

• Fixed by https://github.com/pnlbwh/pnlNipype/commit/3b16a2717aaf4117605b24901b5f9c8e77027fda We are maintaining only one environment36.yml file