Document the suffixes for size values, KB vs KiB, etc

README.md

@@ -45,7 +45,10 @@ transbeam is configured with the following environment variables:
 - `TRANSBEAM_MAX_LIFETIME`: maximum number of days files can be kept
   for (default: 30)
 - `TRANSBEAM_MAX_UPLOAD_SIZE`: maximum size of a fileset being
-  uploaded (default: 16G)
+  uploaded (default: 16G). Size values can be a bare number of bytes,
+  or a number with a suffix. (For example, "K" or "KB" for power-of-10
+  kilobytes, "Ki" or "KiB" for kibibytes - suffixes are not
+  case-sensitive.)
 - `TRANSBEAM_MAX_STORAGE_SIZE`: maximum total size of all files being
   stored by transbeam (default: 64G)
 - `TRANSBEAM_MNEMONIC_CODES`: generate memorable download codes using