Document the PORT field for CSV-format input (zmap#456)

The optional `PORT` argument was added in
7c933df, and documented via comments in
`input.go`, but it was never added to the main documentation in `README.md`.

Co-authored-by: Zakir Durumeric <[email protected]>

Author: dlenskiSB

README.md

@@ -72,17 +72,18 @@ Module specific options must be included after the module. Application specific
 
 ## Input Format
 
-Targets are specified with input files or from `stdin`, in CSV format.  Each input line has three fields:
+Targets are specified with input files or from `stdin`, in CSV format.  Each input line has up to four fields:
 
 ```text
-IP, DOMAIN, TAG
+IP, DOMAIN, TAG, PORT
 ```
 
 Each line must specify `IP`, `DOMAIN`, or both.  If only `DOMAIN` is provided, scanners perform a DNS hostname lookup to determine the IP address.  If both `IP` and `DOMAIN` are provided, scanners connect to `IP` but use `DOMAIN` in protocol-specific contexts, such as the HTTP HOST header and TLS SNI extension.
 
 If the `IP` field contains a CIDR block, the framework will expand it to one target for each IP address in the block.
 
-The `TAG` field is optional and used with the `--trigger` scanner argument.
+The `TAG` field is optional and used with the `--trigger` scanner argument. The `PORT` field is also optional, and acts
+as a per-line override for the `-p`/`--port` option.
 
 Unused fields can be blank, and trailing unused fields can be omitted entirely.  For backwards compatibility, the parser allows lines with only one field to contain `DOMAIN`.
 
@@ -93,7 +94,9 @@ These are examples of valid input lines:
 domain.com
 10.0.0.1, domain.com
 10.0.0.1, domain.com, tag
+10.0.0.1, domain.com, tag, 1234
 10.0.0.1, , tag
+10.0.0.1, , , 5678
 , domain.com, tag
 192.168.0.0/24, , tag