better explain contributing to spreadsheet

Author: ryandawsonuk

README.md

@@ -37,7 +37,7 @@ If you can't access or don't like google sheets then there is a [translation of
 
 Everyone is welcome to contribute, including vendors. Language should be neutral - marketing language will not be accepted.
 
-Changes are welcome by PR but if you wish to add a new platform/column to the matrix, please create your column in a new spreadsheet (following our format) and link from an issue or PR. A maintainer will then update the [master spreadsheet](https://docs.google.com/spreadsheets/d/1nRqjnD7SCMJGmYR2gdZJ84YolLnHAMJwjSG7z7VcM6c/edit?usp=sharing) used to [generate the markdown](https://tabletomarkdown.com/convert-spreadsheet-to-markdown/).
+Changes are welcome by PR or issues - please create a copy of the spreadsheet, link to or upload your copy and explain which parts are changed. Please follow the existing format or raise an issue in advance to suggest changes to the format. On approval a maintainer will then update the [master spreadsheet](https://docs.google.com/spreadsheets/d/1nRqjnD7SCMJGmYR2gdZJ84YolLnHAMJwjSG7z7VcM6c/edit?usp=sharing) used to [generate the markdown](https://tabletomarkdown.com/convert-spreadsheet-to-markdown/).
 
 
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