Add a Go port

It should be roughly at feature parity.

README.adoc

@@ -2,8 +2,8 @@ pdf-simple-sign
 ===============
 :compact-option:
 
-'pdf-simple-sign' is a simple open source PDF signer intended for documents
-generated by Cairo.  As such, it currently comes with some restrictions:
+'pdf-simple-sign' is a simple PDF signer intended for documents produced by
+the Cairo library.  As such, it currently comes with some restrictions:
 
  * the document may not have any forms or signatures already, as they will be
    overwitten
@@ -30,6 +30,10 @@ Runtime dependencies: libcrypto (OpenSSL 1.1 API)
  $ cd builddir
  $ ninja
 
+In addition to the C++ version, also included is a native Go port:
+
+ $ go get janouch.name/pdf-simple-sign/cmd/pdf-simple-sign
+
 Usage
 -----