Write up rst markup

[stevieflow]

write up various conventions used for rst markup

[Bjwebb]

I made a start of this at https://github.com/IATI/IATI-Extra-Documentation/blob/version-1.04/README.rst#markup

[stevieflow]

From #105

• bolean examples are 1/0 (rather than true/false)
• min/multi example XML files - differences between
• reference to the standards in * bold italics*