README: Please do not submit unsolicited pull requests, most likely they will not be merged

[aral-matrix]

Dear users of OpenXLSX,

I would like to clarify this in a place where you are most likely to see it:

If we don't explicitly ask you to submit a pull request, please refrain from creating any, and submit a bug report / feature request instead, please. You are free to link to a fix in the issue, but generally, I do not merge pull requests for various reasons:

• there is a lot of potential for introducing vulnerable code when making major changes, both intentionally and unintentionally. I don't blindly trust anyone's code, and I will not invest time reviewing all those changes
• code written by other people more often than not breaks the coding conventions of a project (variable naming etc)
• the most recent branch is more often than not a development branch, and pull requests submitted against the master branch can be quite messy to merge with changes in the development branch