[skip ci] Fix documentation about snippet import.

Author: damienflament

README.md

@@ -39,16 +39,24 @@ The easiest way to do it is to [import][travis-config-imports] the shared
 configuration snippet:
 
 ```yaml
-import: neimad/travis-python:dev.yml
+import:
+  - source: neimad/travis-python:dev.yml
+    mode: deep_merge_prepend
 ```
 
+**The imported configuration needs to be prepended** and **a deep merge is
+required** to be able use the installed Python distribution in the
+`before_install` and subsequent phases.
+
 Read [the configuration documentation] to understand how it works.
 
 Then, specify the wanted Python version using the `PYTHON` environment
 variable:
 
 ```yaml
-import: neimad/travis-python:dev.yml
+import:
+  - source: neimad/travis-python:dev.yml
+    mode: deep_merge_prepend
 
 env:
   - PYTHON=3.8
@@ -58,7 +66,7 @@ env:
 By default, it will generate a build matrix using the three operating systems
 available on Travis CI (Linux, macOS and Windows).
 
-The Python distribution is installed during the `pre-install` phase and
+The Python distribution is installed during the `before_install` phase and
 available using the `python` program name (whether it is Python 2 or 3).
 
 Behind the scene

doc/Travis_Configuration.md

@@ -36,14 +36,14 @@ the current shell. But as macOS machines use Bash 3.2, sourcing from `stdin` is
 not supported:
 
 ```yaml
-pre-install:
+before_install:
     - source <(curl -sSL $url) # not supported on Bash 3.2
 ```
 
 So, the script is downloaded using `wget` then sourced:
 
 ```yaml
-pre-install:
+before_install:
   - wget https://raw.githubusercontent.com/neimad/travis-python/master/travis-python.bash
   - source travis-python.bash
 ```
@@ -54,7 +54,7 @@ Finally, the Python distribution can be installed in the `$HOME/python`
 directory:
 
 ```yaml
-pre-install:
+before_install:
     - ...
     - install_python $HOME/python $PYTHON
 ```