Installation instructions for Linux are missing the command to check the package signature

The documentation for [Installation on Linux](https://keybase.io/docs/the_app/install_linux) gives the necessary links to check the code signing key, but the subsequent installation instructions do _not_ show how to check that key.

In order to educate users, it would be better to add a section to the doc explaining how to check the package signature:

----

## Check the package signature
To ensure the package you've downloaded was not subverted and is the genuine one provided by Keybase, we encourage you to check its signature prior to installation:

```
# download the detached signature
$ curl -O https://prerelease.keybase.io/keybase_amd64.deb.sig

# Import the keybase public key
# /!\ DOUBLE CHECK the value below here(https://keybase.io/docs/server_security/our_code_signing_key) and here(https://github.com/keybase/client/blob/master/packaging/linux/code_signing_fingerprint)
# If they are not all the same, something odd is happening
$ gpg --recv 222B85B0F90BE2D24CFEB93F47484E50656D16C7

# check the package signature is valid
$ gpg --verify keybase_amd64.deb{.sig,}
gpg: Signature made Tue 05 Dec 2017 07:01:08 PM CET
gpg: using RSA key 222B85B0F90BE2D24CFEB93F47484E50656D16C7
gpg: Good signature from "Keybase.io Code Signing (v1) " [unknown]
```

Open the linked Installation on Linux documentation and review the existing key-signing instructions. Add the requested package-signature section with the provided commands, then verify that the rendered page explains how to check the downloaded package before installation.

Written by the indexing model from the issue text.