Move documentation build instructions to the end, with a separate header

We want to start with the Go 1.5 dependency and build/checkout
instructions.

Also create a separate subsection, to match the future “Building
in/without a container” subsections

README.md

@@ -103,11 +103,6 @@ you'll get an error. You can fix this by either logging in (via `docker login`)
 
 Building
 -
-To build the manual you will need go-md2man.
-```sh
-Debian$ sudo apt-get install go-md2man
-Fedora$ sudo dnf install go-md2man
-```
 To build the `skopeo` binary you need at least Go 1.5 because it uses the latest `GO15VENDOREXPERIMENT` flag. Also, make sure to clone the repository in your `GOPATH` - otherwise compilation fails.
 ```sh
 $ git clone https://github.com/projectatomic/skopeo $GOPATH/src/github.com/projectatomic/skopeo
@@ -124,6 +119,18 @@ You may need to install additional development packages: `gpgme-devel` and `liba
 ```sh
 $ sudo dnf install gpgme-devel libassuan-devel btrfs-progs-devel device-mapper-devel
 ```
+
+### Building documentation
+To build the manual you will need go-md2man.
+```sh
+Debian$ sudo apt-get install go-md2man
+Fedora$ sudo dnf install go-md2man
+```
+Then
+```sh
+$ make docs
+```
+
 Installing
 -
 If you built from source: