docs: how we manage shipped files (#1296)

Signed-off-by: Jan Kowalleck <[email protected]>

Author: jkowalleck

docs/dev/shipped-files.md

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+# Shipped files
+
+Of cause, we ship the build results (JavaScript and type definitions).
+
+We also ship our sources (TypeScript), so that downstream users can work with them directly with own compilers/builders/packers. 
+And we ship all the build instructions we used.  
+Background: our source code requires some special TypeScript compiler options; downstream users might need our special build options, when they work with our shipped sources. (see also: <https://blog.npmjs.org/post/165769683050/publishing-what-you-mean-to-publish.html>)  
+We manage our shipped files in a [`.npmignore`](../../.npmignore) file.