Add some instructions about running integration tests (#3319)

* Add some instructions about running integration tests

* Add missing newline escape

* Suggest running a clean build

integrations/README.md

@@ -0,0 +1,22 @@
+Integration tests can be run with make commands for the
+appropriate backends, namely:
+
+  make test-mysql
+  make test-pgsql
+  make test-sqlite
+
+# Running individual tests
+
+Example command to run GPG test with sqlite backend:
+
+```
+go test -c code.gitea.io/gitea/integrations \
+  -o integrations.sqlite.test -tags 'sqlite' &&
+  GITEA_ROOT="$GOPATH/src/code.gitea.io/gitea" \
+  GITEA_CONF=integrations/sqlite.ini ./integrations.sqlite.test \
+  -test.v -test.run GPG
+```
+
+Make sure to perform a clean build before running tests:
+
+    make clean build