Instructions to help writing reference documentation

[David_Tschumperle (G'MIC staff)]

@grosgood

Markup looks like this (at OldSpace: be3064bc)

|| __Original__ | __Color Vectors__ | __Vector Lengths__ | __Factors__
|  ![orig](img/m_alpha.png) | ![colorvecs](img/m_cpv.png) | ![vectorlen](img/m_nrm.png) | ![factors](img/m_fac.png)

I’ve just detected one whitespace around the |, if you have more (two) there will be still a whitespace in the generated code. Maybe this could explain that ?