Update README.md

Updated fomat_only docs

Author: dellis1972

README.md

@@ -15,9 +15,10 @@ basisu -file foo.png -ktx2
 Adding a `-uastc` might give better results for some compression types.
 
 Once you have the "basis" file you can then transform that into specific formats
-by using the `-unpack` command. PRoviding the `-format_only` flag will produce
+by using the `-unpack` command. Providing the `-format_only` flag will produce
 ONLY that format. Omitting this flag will result in ALL supported formats being
 produced.
+The values for `-format_only` are the numerical values for the [transcoder_texture_format]( https://github.com/BinomialLLC/basis_universal/blob/ad9386a4a1cf2a248f7bbd45f543a7448db15267/transcoder/basisu_transcoder.h#L49). For example ATC with an Alpha channel maps to [cTFATC_RGBA](https://github.com/BinomialLLC/basis_universal/blob/ad9386a4a1cf2a248f7bbd45f543a7448db15267/transcoder/basisu_transcoder.h#L73C3-L73C14) which has a numerical value of `17`.
 
 ```dotnetcli
 basisu -unpack foo.ktx2 -no_ktx -linear -format_only 2