Add issue chains decision

[rsolovjov]

Added decision note.

Issues

Closes #17

[rsolovjov]

I can't understand how will chains be created? Commands to bot or from webhooks? Or they both?
One issue cannot be indifferent chains at the same time?
Therefore, I do not quite understand what types of operations will be used most often.

[sobolevn]

@rsolovjov thanks!

Chains will be created as discussed in #23
One issue can be in multiple chains:

# First task to do -> Next task to do
A -> B
C -> B

In this example B is blocked by A and C.

We will use:

1. Give me next not-blocked issue
2. Build the whole graph of tasks for visualisation