Review general considerations

[pool2win]

Just a way to send over some comments

[bernardoaraujor]

while trying to visualize general_considerations.md

I cannot render this equation to Github on firefox:

\[
    \frac{d({\rm hashrate})}{d({\rm BTC}) = \frac{d_1-d_2}{BTC_1 - BTC_2}
\]

[mcelrath]

@bernardoaraujor fixed, thanks!