Added note about bad ARP entries

When trying to connect to buildslave from dev machine, I experienced
issues with bad entries in the ARP cache. Added a note on how I fixed
it.

Not sure if it was due to an ARP cache bug or a lingering ghost VM.
Hence, I added the note to the end of the README.md to not make two much
noice.

Co-authored-by: Craig Comstock <craig.comstock@northern.tech>
Signed-off-by: Lars Erik Wik <lars.erik.wik@northern.tech>

Author: larsewi

README.md

@@ -288,6 +288,8 @@ Type logout or simply press `CTRL + D` to return to the dev machine:
 logout
 ```
 
+**Didn't work?** Check out [Troubleshooting](#troubleshooting)
+
 From the dev machine, execute the build-remote script:
 
 ```
@@ -492,3 +494,23 @@ the `synced_folder` lines. So something like this:
 -    config.vm.synced_folder ".", "/vagrant",
 +    config.vm.synced_folder ".", "/vagrant", type: "rsync",
 ```
+
+## Troubleshooting
+
+### ssh: connect to host 192.168.56.100 port 22: Protocol not available
+
+If you get the following error when trying to ssh into the buildslave from the dev machine:
+
+```
+ssh: connect to host 192.168.56.100 port 22: Protocol not available
+```
+
+Then I experienced the same issue. Not sure if it was an ARP cache bug or a lingering ghost VM.
+Nevertheless, when I tried to connect, I saw that the ARP cache was updated with a different MAC address then that of the buildslave.
+However, by doing the reverse connection. E.g. (run this command from your host machine):
+
+```
+vagrant ssh buildslave -c "ping -c1 192.168.56.10"
+```
+
+I was able to repopulate the ARP cache with the correct MAC address.