L05DOCBST-41 add README.md

explain how to use this repository as a composer dependency
in a project

README.md

+To use this repository as a dependency in composer.json
+add the following lines:
+```javascript
+    "repositories": [
+        {
+            "type": "gitlab",
+            "url": "https://git.wur.nl/library/php-modules.git"
+        }
+    ],
+```
+This instructs composer to check the library/php-modules repository.
+For composer to be able the correctly access the repository it must
+be told that it is een alternative gitlab server (domain):
+```javascript
+    "config": {
+        "gitlab-domains": ["git.wur.nl"]
+    },
+```
+And composer needs a personal access token to authenticate at the git.wur.nl server.
+Create a personal access token on the git.wur.nl server (in your profile)
+and at it to the file '$HOME/.composer/auth.json':
+```javascript
+{
+    "bitbucket-oauth": {},
+    "github-oauth": {},
+    "gitlab-oauth": {},
+    "gitlab-token": { "git.wur.nl": "my-secret-access-token" }
+}
+```
+Now you can at the dependency:
+```javascript
+    "require": {
+        "library/php-modules": ">=0.0.2"
+    },
+```
+and run 'composer install' or 'composer update'.