Can't install using instructions from the README

[alexdelorenzo]

Hi, cool tool, it works great for my use case.

When I first went to install shelldoc, I ran the following from the README:

$ go get -u github.com/endocode/shelldoc/cmd/shelldoc
    go: downloading github.com/endocode/shelldoc v0.0.0-20190801062746-88c3b43825b0
    go get: github.com/endocode/shelldoc@v0.0.0-20190801062746-88c3b43825b0 requires
    gopkg.in/russross/blackfriday.v2@v2.0.1: parsing go.mod:
    module declares its path as: github.com/russross/blackfriday/v2
      but was required as: gopkg.in/russross/blackfriday.v2

Which failed with the error above. I had to pull the repository and run the Makefile in order to build a copy of shelldoc.

During that build, one of the self-tests that runs the go get command from the README failed, as well:

$ make
git describe  --always --tags --abbrev=7 HEAD > VERSION
cd cmd/shelldoc && go build -ldflags '-X github.com/endocode/shelldoc/pkg/version.versionString=v0.2-4-g88c3b43'
Running self-test of README.md and evaluating XML output with xmllint...
SHELLDOC: doc-testing "README.md" ...
 CMD (1): echo Hello                                ?  Hello                      :  PASS (match)
 CMD (2): go get -u github.com/endocode/shelldo...  ?  ...                        :  FAIL (execution failed)
 CMD (3): export GREETING="Hello World"             ?  (no response expected)     :  PASS (execution successful)
 CMD (4): echo $GREETING                            ?  Hello World                :  PASS (match)
 CMD (5): echo Hello && false                       ?  Hello                      :  PASS (match)
 CMD (6): (exit 2)                                  ?  (no response expected)     :  PASS (execution successful)
FAILURE: 6 tests - 5 successful, 1 failures, 0 errors
make: *** [Makefile:15: selftest] Error 1

Here's my setup:

$ go version
go version go1.17.3 linux/amd64