Update README and Makefile

[michplunkett]

Describe your changes

Added missing information to README and a missing command to the Makefile.

Checklist before requesting a review

• The code runs successfully.

michaelp@MacBook-Air-18 golang-scripting % ls
LICENSE		Makefile	README.md	cmd		go.mod		go.sum
michaelp@MacBook-Air-18 golang-scripting % make parse-slack-data
go run ./cmd/slackMessageParser/main.go
2024/07/16 00:08:50 the ./SlackMessages directory does not exist
exit status 1
make: *** [parse-slack-data] Error 1
michaelp@MacBook-Air-18 golang-scripting % mkdir -p ./SlackMessages
michaelp@MacBook-Air-18 golang-scripting % make parse-slack-data
go run ./cmd/slackMessageParser/main.go
2024/07/16 00:11:16 could not find *.json files in the ./SlackMessages directory
exit status 1
michaelp@MacBook-Air-18 golang-scripting % cp ~/Downloads/Archive/* ./SlackMessages
michaelp@MacBook-Air-18 golang-scripting % make parse-slack-data
go run ./cmd/slackMessageParser/main.go
michaelp@MacBook-Air-18 golang-scripting % ls
LICENSE			Makefile		README.md		SlackMessages		cmd			go.mod			go.sum			slack_records.csv
michaelp@MacBook-Air-18 golang-scripting %