Add a proviso for working with others (see #97)

en/lesson-02.md

@@ -69,6 +69,15 @@ can edit our examples using [Overleaf](https://www.overleaf.com), one of the
 major websites for LaTeX online. There are of course others:
 [Papeeria](https://papeeria.com/) is an example.
 
+## Working with others
+
+If you are planning to send your LaTeX sources to destinations which process
+them, such as publishers, conference organisers or pre-print servers
+(e.g. arXiv), you should check what restrictions they impose. In particular,
+the availability of Unicode engines and/or engines that call external tools
+(such as for working on bibliographies) and up-to-date packages may not be
+guaranteed. It's best to check first and write later!
+
 ## Exercise
 
 Get yourself set up with a local LaTeX installation _or_ an account with