doc: Adopters page (#254)

Adds page with full list of `avo` adopters. Linked to from README.

Updates #101

internal/cmd/docgen/templates/readme.tmpl

@@ -20,6 +20,7 @@ For more about `avo`:
 * Introductory talk ["Better `x86` Assembly Generation with Go"](https://www.youtube.com/watch?v=6Y5CZ7_tyA4) at [dotGo 2019](https://2019.dotgo.eu/) ([slides](https://speakerdeck.com/mmcloughlin/better-x86-assembly-generation-with-go))
 * [Longer tutorial at Gophercon 2019](https://www.youtube.com/watch?v=WaD8sNqroAw) showing a highly-optimized dot product ([slides](https://speakerdeck.com/mmcloughlin/better-x86-assembly-generation-with-go-gophercon-2019))
 * Watch [Filippo Valsorda](https://filippo.io/) live code the [rewrite of `filippo.io/edwards25519` assembly with `avo`](https://vimeo.com/679848853)
+* Explore [projects using `avo`](doc/adopters.md)
 * Discuss `avo` and general Go assembly topics in the [#assembly](https://gophers.slack.com/archives/C6WDZJ70S) channel of [Gophers Slack](https://invite.slack.golangbridge.org/)
 
 _Note: APIs subject to change while `avo` is still in an experimental phase. You can use it to build [real things](examples) but we suggest you pin a version with your package manager of choice._
@@ -106,13 +107,15 @@ Popular projects using `avo`:
 
 {{ range .Packages.Top 10 -}}
 {{ if lt .Metadata.Stars 100 }}{{break}}{{ end -}}
-<img src="https://github.com/{{ .Repository.Owner }}.png?size=24" width="24" height="24" hspace="4" valign="middle" /> [{{ .Repository.Owner }} / **{{ .Repository.Name }}**]({{ .Repository.URL }})
+{{ avatar .Repository.Owner }} [{{ .Repository.Owner }} / **{{ .Repository.Name }}**]({{ .Repository.URL }})
 {{- if .SubPackage }} / **{{ .SubPackage}}**{{ end }}
 :star: {{ .Metadata.Stars }}
 > {{ .Metadata.Description }}
 
 {{ end -}}
 
+See the [full list of projects using `avo`](doc/adopters.md).
+
 ## Contributing
 
 Contributions to `avo` are welcome: