doc: add steps to fix read-only fs

PR-URL: #2231
Fixes: #2230
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rod Vagg <rod@vagg.org>

doc/jenkins-guide.md

@@ -13,6 +13,7 @@ A guide on maintaining Node.js' Test and Release Jenkins clusters
   * [Out of memory](#out-of-memory)
   * [Out of space](#out-of-space)
   * [General issues with Jenkins agent: "normal machines" edition](#general-issues-with-jenkins-agent-normal-machines-edition)
+  * [Read-only filesystem](#read-only-filesystem)
   * [Restart the machine](#restart-the-machine)
   * [Fixing machines with Docker](#fixing-machines-with-docker)
   * [IDK what to do](#idk-what-to-do)
@@ -200,6 +201,24 @@ launchctl start org.nodejs.osx.jenkins
 ~iojs/start.sh
 ```
 
+### Read-only filesystem
+
+Sometimes a failure in the filesystem will cause it to enter a read-only state.
+If that happens, follow the steps below:
+
+```bash
+touch foo                      # to confirm system is read-only, don't proceed if this succeeds
+sudo df                        # to determine the device for `/`
+sudo e2fsck -y /dev/mmcblk0p2  # replace mmcblk0p2 with proper device for `/`
+```
+
+After running the steps above, follow instructions below to 
+[restart the machine](#restart-the-machine).
+
+Note: the occurrence of read-only root filesystem indicates there's probably a
+more concerning issue going on. Remember to open an issue on nodejs/build to 
+investigate further, as it might be required to reprovision the machine.
+
 ### Restart the machine
 
 Sometimes something weird happens, and it's easier to just reboot the