added a section to the readme about the collapse_features setting, why it exist and how to use it

.kdev4/debcargo-conf.kdev4

@@ -0,0 +1,2 @@
+[Project]
+VersionControlSupport=kdevgit

README.rst

@@ -226,6 +226,30 @@ conditions for (2), e.g. https://github.com/hsivonen/simd/issues/25
 obvious so documentation is less necessary, and dependent crates all do it
 correctly already.)
 
+Setting collapse_features in debcargo.conf
+------------------------------------------
+
+Rust and Debian have a two different levels of abstraction when handling dependencies and the
+relationship between them. In rust the lowest level is a feature, and in Debian it's the binary
+package.
+
+This means that the following dependency chain is not a problem in rust:
+
+- crate A with feature AX depends on crate B with feature BY
+- crate B with feature BX depends on crate A with feature AY
+
+This is a perfectly valid situation in the rust+cargo ecosystem. Notice that
+there is no dependency cycle on the per-feature level, and this is enforced by
+cargo; but if collapse_features is used then package A+AX+AY would cyclicly
+depend on package B+BX+BY.
+
+This is reflected in the Debian packages by producing `Provides` lines for all combinations
+of features, and this can become a quite large section.
+
+Setting `collapse_features = true` in debcargo.toml removes this behaviour and is recommended,
+but it can lead to dependency cycles of debian packages, if that happens those must be
+broken up by having some or all of the packages set this feature to false.
+
 Changed orig tarballs
 ---------------------
 

debcargo-conf.kdev4

@@ -0,0 +1,4 @@
+[Project]
+CreatedFrom=
+Manager=KDevCustomBuildSystem
+Name=debcargo-conf