add ability to force specific mime-types to text through env var

docs/howto/force-mimetype-to-text.md

@@ -0,0 +1,21 @@
+# Force mime type to text
+
+The following `.gitignore` is for example considered
+binary by dotdrop since its mime type is `application/x-wine-extension-ini`
+
+```
+[user]
+        name = user
+        email = user@example.com
+
+[credential]
+        helper = cache
+```
+
+Dotdrop can be forced to consider specific mime types as text.
+Set the following environment variable:
+```bash
+export DOTDROP_MIME_TEXT=application/x-wine-extension-ini
+```
+
+see [environment variables](../usage.md#environment-variables)