add docs

src/table/dataframes_interface.jl

@@ -225,7 +225,21 @@ end
 manipulate(df::DTable, c::ColumnIndex; copycols::Bool, keeprows::Bool, renamecols::Bool) =
     manipulate(df, Int[index(df)[c]], copycols=copycols, keeprows=keeprows, renamecols=renamecols)
 
-# Copied as is from DataFrames.jl
+
+"""
+    select(df::DTable, args...; copycols::Bool=true, renamecols::Bool=true)
+
+Create a new DTable that contains columns from `df` specified by `args` and return it.
+The result is guaranteed to have the same number of rows as df, except when no columns
+are selected (in which case the result has zero rows).
+
+This operation is supposed to provide the same functionality and syntax as `DataFrames.select`,
+but for DTable input. Most cases should be covered and the output should be exactly the
+same as one obtained using DataFrames. In case of output differences or `args` causing errors
+please file an issue with reproduction steps and data.
+
+Please refer to DataFrames documentation for more details on usage.
+"""
 select(df::DTable, @nospecialize(args...); copycols::Bool=true, renamecols::Bool=true) =
     manipulate(df, map(x -> broadcast_pair(df, x), args)...,
         copycols=copycols, keeprows=true, renamecols=renamecols)