README: expand installation section more

This adds a small block for the go.mod bit, and describes how kgo
relates to the repo.

It is common for repos to provide one package, and thus the repo root is
the package. It's not always the rule, though: repos with go-xyz names
may have a go-xyz/xyz package, and large repos (aws-sdk-go) have dozens
of packages.

Closes #183.

README.md

@@ -53,7 +53,10 @@ released independently. The main client is in franz-go. Plugins are released
 from `plugin/{plugin}`. The raw-protocol package is released from `pkg/kmsg`,
 and the admin package is released from `pkg/kadm`.
 
-To use just the main client,
+The main client is located in the package `github.com/twmb/franz-go/pkg/kgo`,
+while the root of the project is at `github.com/twmb/franz-go`. There are
+a few extra packages within the project, as well as a few sub-modules. To
+use the main kgo package,
 
 ```
 go get github.com/twmb/franz-go
@@ -71,6 +74,16 @@ To use kadm,
 go get github.com/twmb/franz-go/pkg/kadm
 ```
 
+As an example, your require section in go.mod may look like this:
+
+
+```
+require (
+	github.com/twmb/franz-go v1.7.0
+	github.com/twmb/franz-go/pkg/kmsg v1.1.0
+)
+```
+
 ## Getting started
 
 Here's a basic overview of producing and consuming: