Read godoc in vim using Shift K

Add the following to your ~/.vim/ftplugin/go.vim (the key here is to use <cfile> not <cword>)

fun! ReadMan()
  " Assign current file under cursor to a script variable:
  let s:man_word = expand('<cfile>')
  :exe ":Godoc " . s:man_word
endfun
" Map the K key to the ReadMan function:
map K :call ReadMan()<CR>

=======UPDATE========
Had a better way for this:

1. Create a file in your ~/bin called godoc.sh with the following content:
#!/bin/sh
fullpkg=`gawk -v "cmd=$1" '
{ FS="/" }
{
        if ($NF==cmd || $0 ==cmd){
                printf("%s",$0);
                exit;
        }
}' ~/bin/gopkg-list.txt`
echo -n $fullpkg

2. Create a text file in ~/bin/gopkg-list.txt with the following contents:

log/syslog

unicode/utf8

unicode/utf16

testing

hash/crc64

hash/adler32

hash/fnv

hash/crc32

archive/zip

archive/tar

expvar

mime

reflect

crypto/des

crypto/sha1

crypto/subtle

crypto/aes

crypto/md5

crypto/cipher

crypto/rand

crypto/ecdsa

crypto/tls

crypto/sha512

crypto/x509

crypto/sha256

crypto/elliptic

crypto/hmac

crypto/rsa

crypto/x509/pkix

crypto/rc4

crypto/dsa

flag

strings

sync

os/user

os/signal

os/exec

os

fmt

syscall

time

runtime/pprof

runtime/cgo

runtime/debug

runtime/race

net/http/httputil

net/http/cookiejar

net/http/fcgi

net/http/pprof

net/http/cgi

net/http/httptest

net/rpc/jsonrpc

net/mail

net/http

net/textproto

net/url

net/rpc

net/smtp

io/ioutil

math

database/sql/driver

database/sql

mime/multipart

bytes

errors

sort

bufio

encoding

log

path/filepath

index/suffixarray

net

crypto

sync/atomic

go/printer

go/doc

go/scanner

go/ast

go/parser

go/token

go/build

go/format

unicode

image

io

runtime

html/template

regexp

html

container/ring

container/list

container/heap

hash

path

regexp/syntax

text/template/parse

text/tabwriter

text/template

text/scanner

strconv

testing/quick

testing/iotest

compress/flate

compress/zlib

compress/lzw

compress/bzip2

compress/gzip

image/color

image/draw

image/color/palette

image/png

image/gif

image/jpeg

encoding/base64

encoding/base32

encoding/xml

encoding/asn1

encoding/pem

encoding/ascii85

encoding/csv

encoding/hex

encoding/json

encoding/binary

encoding/gob

debug/pe

debug/plan9obj

debug/dwarf

debug/elf

debug/gosym

debug/macho

math/rand

math/big

math/cmplx

3. In your ~/.vim/ftplugin/go.vim, add the following:

fun! ReadMan()

  " Assign current file under cursor to a script variable:

  let s:man_word = expand('<cword>')

  let s:full_word = system('~/bin/godoc.sh '. s:man_word)

  :exe ":Godoc " . s:full_word

endfun

" Map the K key to the ReadMan function:

map K :call ReadMan()<CR>

This will take the keyword under cursor, expanded using the shell script, and call :Godoc on it. This takes care of subpackages