README: expand on the pkg-config compatibility promise

A bunch of CMake developers, who apparently did not comprehend the previous
compatibility promise statement, decided to passive-aggressively demand we fix
their non-bug anyway.

Clarify in more detail what level of pkg-config compatibility we provide, and
mention that such passive-aggressive behaviour is considered a CoC violation.

Fixes #228.

README.md

@@ -60,6 +60,10 @@ Doing so is discouraged by the [freedesktop tutorial][fd-tut] anyway.
 
 ## compatibility with pkg-config
 
+I really hate that I have to have this section, I like being a nice person, but we
+unfortunately have to say this because otherwise we get passive-aggressive people who
+try to argue with us about what pkg-config compatibility means.
+
 We do not provide bug-level compatibility with pkg-config.
 
 What that means is, if you feel that there is a legitimate regression versus pkg-config,
@@ -67,6 +71,17 @@ do let us know, but also make sure that the .pc files are valid and follow the r
 the [pkg-config tutorial][fd-tut], as most likely fixing them to follow the specified
 rules will solve the problem.
 
+Additionally, **we do not consider pkgconf doing what you tell it to do, when pkg-config
+fails to do so, to be a bug**.
+
+If, for example, you use environment variables such as `PKG_CONFIG_SYSTEM_[INCLUDE|LIBRARY]_PATH`
+and then find yourself surprised that `pkgconf` is stripping `-I` and `-L` flags relating
+to those paths, it's not a `pkgconf` problem -- `pkgconf` is doing exactly what you told
+it to do.
+
+We will reject bugs like this, and if you choose to violate our [Code of Conduct](CODE_OF_CONDUCT.md)
+by demanding we fix your non-bug, we will fix the problem by banning you from the repo instead.
+
 ## compiling `pkgconf` and `libpkgconf` on UNIX
 
 pkgconf is basically compiled the same way any other autotools-based project is