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 (Add parameter for assuming minor allele is the effect allele #193)
• dbSNP versions (SNPlocs.Hsapiens.dbSNP156.GRCh37 #167)
• MAF vs EAF (Inquiry on Converting MAF to EAF Using MungeSumstats #189)
• Data from differing ancestries (MungeSumstats on GWAS data from non-Europeans #190) (Non-European populations #143)
• Liftover and using local chain files (Add local chain file option for MungeSumstats::liftover # enhancement #183)
• Filtering of non-bi-allelic SNPs (Large number of non-biallelic SNPs #111) - also set default to keeping non-bi-allelic SNPs when this is added.
• Flipping MAF with multi-allelic SNPs (Allele frequency cant be flipped for multi-allelic variants. #165)
• A2 is the effect allele not A1 (Option to specify effect allele as A1 instead.  #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