Verify "go mod tidy" in CI

Previously, there was no verification that it was possible to run
"go mod tidy" and "go mod vendor" in projects that depend on errors.
This commit adds a CI step to check that these "go mod" commands can
run successfully.

.github/workflows/ci.yaml

@@ -30,3 +30,6 @@ jobs:
 
     - name: Test (${{ matrix.go }})
       run: go test ./...
+
+    - name: Tidy (${{ matrix.go }})
+      run: '[[ `go version` < "go version go1.15.10" ]] || go mod tidy'

README.md

@@ -58,6 +58,8 @@ older version of the package.
 
 ## How to use
 
+This library works best with go ≥ 1.15.10. It works with 1.15.9 but breaks `go mod tidy` and `go mod vendor`.
+
 - construct errors with `errors.New()`, etc as usual, but also see the other [error leaf constructors](#Available-error-leaves) below.
 - wrap errors with `errors.Wrap()` as usual, but also see the [other wrappers](#Available-wrapper-constructors) below.
 - test error identity with `errors.Is()` as usual.