document guarantee that getmetadata returns alias not copy (#169)

src/write.jl

@@ -31,11 +31,16 @@ end
 """
     Arrow.getmetadata(x) => Dict{String, String}
 
-Retrieve any metadata (as a `Dict{String, String}`) attached to an object.
+Retrieve any metadata (as a `Dict{String, String}`) attached to `x`.
 
 Metadata may be attached to any object via [`Arrow.setmetadata!`](@ref),
 or deserialized via the arrow format directly (the format allows attaching metadata
 to table, column, and other objects).
+
+Note that this function's return value directly aliases `x`'s attached metadata 
+(i.e. is not a copy of the underlying storage). Any method author that overloads 
+this function should preserve this behavior so that downstream callers can rely
+on this behavior in generic code.
 """
 getmetadata(x, default=nothing) = get(OBJ_METADATA, x, default)