Add docstring for Variation

src/Variation.jl

@@ -1,3 +1,25 @@
+"""
+    Variation{S<:BioSequence,T<:BioSymbol}
+
+A single change to a biological sequence. A general wrapper that can represent a
+sequence-specific [`Substitution`](@ref), [`Deletion`](@ref) or [`Insertion`](@ref).
+`Variation` is more robust than [`Edit`](@ref), due to inclusion of the reference sequence
+and built-in validation.
+
+# Constructors
+
+    Variation(ref::S, e::Edit{S,T}) where {S<:BioSequence,T<:BioSymbol}
+    Variation(ref::S, edit::AbstractString) where {S<:BioSequence}
+
+Generally speaking, the `Edit` constructor should be avoided to ensure corectness: use of
+[`variations(::Variant)`](@ref) is encouraged, instead.
+
+Constructing a `Variation` from an `AbstractString` will parse the from `edit` using the
+following syntax:
+- Substitution: `"<REFBASE><POS><ALTBASE>"`, e.g. `"G16C"`
+- Deletion: `"Δ<STARTPOS>-<ENDPOS>"`, e.g. `"Δ1-2"`
+- Insertion: `"<POS><ALTBASES>"`, e.g. `"11T"`
+"""
 struct Variation{S<:BioSequence,T<:BioSymbol}
     ref::S
     edit::Edit{S,T}