Add installing from source code guide

Author: gustavosbarreto

docs/user-manual/installing.md

@@ -1,12 +1,10 @@
-ShellHub Agent is designed to run on all your systems: physical and virtual servers, even IoT/edge devices.
-
 The best way to install ShellHub Agent is with our automatic one-line installation script, which works with all Linux distributions that have Docker installed and properly set up.
-
-If you want to install ShellHub without Docker you can try to build by yourself or if you are targeting embedded devices checkout our [Yocto Project support layer for ShellHub](https://github.com/shellhub-io/meta-shellhub).
+If you want to install ShellHub without Docker you can try to [install from source code](#installing-from-source-code) or if you are targeting embedded devices checkout our [Yocto Project support layer for ShellHub](https://github.com/shellhub-io/meta-shellhub).
 
 ## One-line installation script
 
-!!! info "Docker installed and properly set up on device is required"
+!!! info "Prerequisites"
+	Docker installed and properly set up on device is required.
 	Follow the [Docker Install Instructions](http://docs.docker.com/installation/) for your distro/platform.
 	You should always use the latest docker version. The minimum supported Docker version is `18.06`.
 
@@ -18,3 +16,27 @@ After running the command on your device, you'll need to accept the device on th
 Go to _Devices_ -> _Pending_ locate your device and click _Accept_ button.
 
 After your device is enrolled, it becomes available in _Device List_ page.
+
+## Installing from source code
+
+!!! info "Prerequisites"
+	ShellHub Agent requires Go 1.14 to compile, please refer to the [official documentation](https://golang.org/doc/install) for how to install Go in your system.
+
+### Checkout source code
+
+First checkout the latest stable version (*{{ shellhub.latest_version }}*) of ShellHub as follows: 
+
+```
+git clone -b {{ shellhub.latest_version }} https://github.com/shellhub-io/shellhub.git shellhub
+```
+
+### Building
+
+Execute the following command to build ShellHub Agent:
+
+```
+cd shellhub/agent
+go build
+```
+
+Use `file ./agent` to check if executable was built.