Book: Add Proposals description

Author: flip1995

book/src/development/proposals/README.md

@@ -1 +1,11 @@
 # Proposals
+
+This chapter is about accepted proposals for changes that should be worked on in
+or around Clippy in the long run.
+
+Besides adding more and more lints and improve the lints that Clippy already
+has, Clippy is also interested in making the experience of its users, developers
+and maintainers better over time. Projects that address bigger picture things
+like this usually take more time and it is useful to have a proposal for those
+first. This is the place where such proposals are collected, so that we can
+refer to them when working on them.