docs(README.md): Add abspath, mod, and ignoregenerated flag docs (#220)

Noticed that some useful flags were not documented in the README.md. I've added the these in what I think are the relevant sections. I did a minimal explanation of the flags, but can expand on them.

I erred on the side of not expanding on -ignoregenerated for example because the regex might change in the future, and documentation tends to be a forgotten update...

README.md

@@ -36,6 +36,10 @@ takes no arguments.
 The `-blank` flag enables checking for assignments of errors to the
 blank identifier. It takes no arguments.
 
+The `-abspath` flag prints the absolute paths to files with unchecked errors.
+
+The `-mod` flag sets the module download mode to use: `readonly` or `vendor`.
+
 ### go/analysis
 
 The package provides `Analyzer` instance that can be used with
@@ -120,6 +124,8 @@ specified for it. To disable this, specify a regex that matches nothing:
 The `-ignoretests` flag disables checking of `_test.go` files. It takes
 no arguments.
 
+The `-ignoregenerated` flag disables checking of generated source code. It takes no arguments.
+
 ## Exit Codes
 
 errcheck returns 1 if any problems were found in the checked files.