clarify that input with an alpha channel requires additional computation (closes: #142)

README.md

@@ -28,10 +28,10 @@ The following table shows how img2pdf handles different input depending on the
 input file format and image color space.
 
 | Format                                | Colorspace                     | Result        |
-| -------------------- | ------------------------------ | ------------- |
+| ------------------------------------- | ------------------------------ | ------------- |
 | JPEG                                  | any                            | direct        |
 | JPEG2000                              | any                            | direct        |
-| PNG (non-interlaced) | any                            | direct        |
+| PNG (non-interlaced, no transparency) | any                            | direct        |
 | TIFF (CCITT Group 4)                  | monochrome                     | direct        |
 | any                                   | any except CMYK and monochrome | PNG Paeth     |
 | any                                   | monochrome                     | CCITT Group 4 |

src/img2pdf.py

@@ -1823,8 +1823,8 @@ def read_images(rawdata, colorspace, first_frame_only=False, rot=None):
 
                 smaskidat, _, _ = to_png_data(a)
                 logger.warning(
-                    "Image contains an alpha channel which will be stored "
-                    "as a separate soft mask (/SMask) image in PDF."
+                    "Image contains an alpha channel. Computing a separate "
+                    "soft mask (/SMask) image to store transparency in PDF."
                 )
             elif color in [Colorspace.P, Colorspace.PA] and iccp is not None:
                 # PDF does not support palette images with icc profile