update mpdf lib version from 6 to 8

doc/composer.md

@@ -0,0 +1,33 @@
+# Composer Knowledge base
+
+## upgrade packages
+
+[upgrade.txt](./upgrade.txt)
+
+## use newer github tokens
+
+Edit the composer authentication configuration file ~/.composer/auth.json.
+```bash
+nano ~/.composer/auth.json
+```
+Then replace the following.
+```json
+{
+"github-oauth": {
+"github.com": "ghp_[YOUR-PERSONAL-TOKEN]"
+}
+}
+```
+With this (basic auth):
+
+```json
+{
+  "http-basic": {
+    "github.com": {
+      "username": "[YOUR-GITHUB-USERNAME]",
+      "password": "ghp_[YOUR-PERSONAL-TOKEN]"
+    }
+  }
+}
+```
+