Add FAQ of common queries to documentation

[Al-Murphy]

I have noted some repeated questions so I think an FAQ section would be appropriate to cover these and some of the important aspects of MSS like:

• How to generally use MSS
• Effect allele vs minor allele (https://github.com/Al-Murphy/MungeSumstats/issues/193)
• dbSNP versions (https://github.com/Al-Murphy/MungeSumstats/issues/167)
• MAF vs EAF (https://github.com/Al-Murphy/MungeSumstats/issues/189)
• Data from differing ancestries (https://github.com/Al-Murphy/MungeSumstats/issues/190) (https://github.com/Al-Murphy/MungeSumstats/issues/143)
• Liftover and using local chain files (https://github.com/Al-Murphy/MungeSumstats/issues/183)
• Filtering of non-bi-allelic SNPs (https://github.com/Al-Murphy/MungeSumstats/issues/111)(https://github.com/Al-Murphy/MungeSumstats/issues/124) - also set default to keeping non-bi-allelic SNPs when this is added.
• Flipping MAF with multi-allelic SNPs (https://github.com/Al-Murphy/MungeSumstats/issues/165)
• A2 is the effect allele not A1 (https://github.com/Al-Murphy/MungeSumstats/issues/160)
• Ambiguous naming of allele columns like A1/A2 - means different things to different people - the infer_effect_column(), function here should catch cases where the naming is ambiguous of the effect/non-effect allele such as A1/A2. See