Put install instructions of test tools into separate docs/ markdown file

docs/develop/Documentation.md

@@ -8,6 +8,9 @@ To preview local changes, first install MkDocs
 pip3 install --user mkdocs
 ```
 
+If `mkdocs` can't be found after the installation, the $PATH might have not
+be set correctly yet. Try opening a new terminal session.
+
 
 Then go to the build directory and run
 

docs/develop/Setup.md

@@ -0,0 +1,46 @@
+# Setup Test Environment
+
+To test changes and contribute to Nominatim you should be able to run
+the test suite(s). For many usecases it's enough to create a Vagrant
+virtual machine (see `VAGRANT.md`), import one small country into the
+database.
+
+## Prerequisites
+
+Nominatim supports a range of PHP versions and PHPUnit versions also
+move fast. We try to test against the newest stable PHP release and
+PHPUnit version even though we expect many Nominatim users will install
+older version on their production servers.
+
+#### Ubuntu 20
+
+    sudo apt-get install -y phpunit php-codesniffer php-cgi
+    pip3 install --user behave nose
+
+#### Ubuntu 18
+
+    pip3 install --user behave nose
+
+    sudo apt-get install -y composer php-cgi php-cli php-mbstring php-xml zip unzip
+
+    composer global require "squizlabs/php_codesniffer=*"
+    sudo ln -s ~/.config/composer/vendor/bin/phpcs /usr/bin/
+
+    composer global require "phpunit/phpunit=8.*"
+    sudo ln -s ~/.config/composer/vendor/bin/phpunit /usr/bin/
+
+
+#### CentOS 7 or 8
+
+    sudo dnf install -y php-dom php-mbstring
+    pip3 install --user behave nose
+
+    composer global require "squizlabs/php_codesniffer=*"
+    sudo ln -s ~/.config/composer/vendor/bin/phpcs /usr/bin/
+
+    composer global require "phpunit/phpunit=^7"
+    sudo ln -s ~/.config/composer/vendor/bin/phpunit /usr/bin/
+
+## Run tests, code linter, code coverage
+
+See `README.md` in `test` subdirectory.