Added documentation

docs/src/fields.md

@@ -0,0 +1,25 @@
+
+# Lattice fields
+
+The module `Fields` include simple routines to define a few typical
+fields. Fields are simple `CuArray` types with special memory
+layout. A field always has an associated elemental type (i.e. for 
+gauge fields `SU3`, for scalar fields `Float64`). We have:
+- scalar fields: One elemental type in each spacetime point.
+- vector field: One elemental type at each spacetime point and
+  direction.
+- `N` scalar fields: `N` elemental types at each spacetime point.
+
+For all these fields the spacetime point are ordered in memory
+according to the point-in-block and block indices (see
+[`SpaceParm`](@ref)). An execption is the [`scalar_field_point`](@ref)
+fields. 
+
+## Initialization
+
+```@docs
+scalar_field
+vector_field
+nscalar_field
+scalar_field_point
+```