README: add info for Linux install

cf pypa/pipenv#3902

README.md

@@ -8,7 +8,7 @@ In this document we use the fictional term "spiffy" as an example of a template
 
 In this README we first focus on the workflow to update an existing template, then we describe how to create a new one.
 
-> **Note:** 
+> **Note:**
 >
 > For additional information about how specific similar templates are defined, see [notes.md](/notes.md).
 
@@ -24,6 +24,8 @@ In this README we first focus on the workflow to update an existing template, th
 1. First install the dependencies:
 ```
 pipenv install
+# Note if you are on Linux please run this before to update the lock file (doit have different dependencies on Mac and Linux)
+pipenv lock --keep-outdated
 ```
 2. You will need to have your user added to the `platformsh-templates` github organisation in order to be able to push to these repositories.