doc: improved doc about the build from sources. (#1162)

Author: ldez

CONTRIBUTING.md

@@ -35,7 +35,7 @@ owners to license your work under the terms of the [MIT License](LICENSE).
 
 Requirements:
 
-- `go` v1.12+
+- `go` v1.14+
 - environment variable: `GO111MODULE=on`
 
 First, you have to install [GoLang](https://golang.org/doc/install) and [golangci-lint](https://github.com/golangci/golangci-lint#install).

docs/content/installation/_index.md

@@ -42,11 +42,19 @@ pkg install lego
 
 Requirements:
 
-- `go` v1.12+
+- `go` v1.14+
 - environment variable: `GO111MODULE=on`
 
 To install the latest development version from sources, just run:
 
 ```bash
 go get -u github.com/go-acme/lego/v3/cmd/lego
 ```
+
+or
+
+```bash
+git clone [email protected]:go-acme/lego.git
+make        # tests + doc + build
+make build  # only build
+```