Bug 1522090 - Add info to troubleshoot './mach bootstrap'. r=Ola

Add info to troubleshoot './mach bootstrap'

Differential Revision: https://phabricator.services.mozilla.com/D19271

--HG--
extra : moz-landing-system : lando

devtools/docs/getting-started/build.md

@@ -63,6 +63,8 @@ Then run this:
 ./mach build
 ```
 
+Please note, if this fails it might be possible you need to run the `bootstrap.py` script first. Download the [bootstrap.py script](https://hg.mozilla.org/mozilla-central/raw-file/default/python/mozboot/bin/bootstrap.py) and save it in your project directory. Then run `python bootstrap.py` and follow the prompted steps.
+
 **Note:** if using Windows, you might need to type the commands without the `./`:
 
 ```bash