docs: add a common_errors file to hopefully help avoid debugging the …

docs/common_errors.md

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+# Common Errors
+
+## Javascript
+
+### Error when writing tests for components in a project importing adhocracy4
+
+When writing tests with Jest (and/or testing-libray) in a project which uses adhocracy4 and the component being
+tested imports a newly developed component from adhocracy4 you might encounter the following
+issue:
+
+```
+Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: undefined.
+```
+
+One possible cause is that you used `npm link` to link to your local a4.
+If you get the above error try deleting your `node_modules`
+folder (e.g. by running `make clean`) and then properly install a4 via `npm
+install liqd/adhocracy4#<name of your branch or tag>` (assuming you already
+pushed your changes in a4, otherwise create a branch).
+It doesn't seem to be enough to install it from a local folder via `npm install <path to a4>` for some reason.