Update troubleshooting.md (#242)

* Update troubleshooting.md

I've come across this issue several times and after searching the docs I end up on [this](roots/trellis#1117) github issue where this plugin is suggested by ianjamieson. I totally understand if this isn't the right place for this to go, or if you would rather leave it out of the documentation, but I know that for me it's really nice to be able to quickly peak at this troubleshooting section for quick things to try. Happy to re-phrase, or offer more detail if necessary.

* Update troubleshooting.md

Added note that this was only for mac users

docs/trellis/master/troubleshooting.md

@@ -175,3 +175,11 @@ $ ssh -v root@12.34.56.78
 ### `Ciphers`, `KexAlgorithms`, or `MACs`
 
 The `sshd` role will most likely cause your SSH server to discontinue using some older and weaker protocols. If your connections involve older systems that do not support the stronger protocols configured by the `sshd` role, see [`Ciphers`, `KexAlgorithms`, and `MACs`](https://github.com/roots/trellis/tree/master/roles/sshd#ciphers-kexalgorithms-and-macs) for how to add back in any protocols you need.
+
+### NET::ERR_CERT_INVALID for Mac Users
+If you are running Trellis on MacOS and receiving a `NET::ERR_CERT_INVALID` error on your local dev domain, you may want to try using the `vagrant-trellis-cert` plugin using the following commands:
+```bash
+cd trellis
+vagrant plugin install vagrant-trellis-cert
+vagrant trellis-cert trust
+```