add golangci-lint configuration

It's mostly based on neofs-http-gate configuration, but has exhaustive
disabled and some golint-specific warnings enabled (some of which are
important for us).

.golangci.yml

@@ -0,0 +1,58 @@
+# This file contains all available configuration options
+# with their default values.
+
+# options for analysis running
+run:
+  # timeout for analysis, e.g. 30s, 5m, default is 1m
+  timeout: 5m
+
+  # include test files or not, default is true
+  tests: true
+
+# output configuration options
+output:
+  # colored-line-number|line-number|json|tab|checkstyle|code-climate, default is "colored-line-number"
+  format: tab
+
+# all available settings of specific linters
+linters-settings:
+  exhaustive:
+    # indicates that switch statements are to be considered exhaustive if a
+    # 'default' case is present, even if all enum members aren't listed in the
+    # switch
+    default-signifies-exhaustive: true
+  govet:
+    # report about shadowed variables
+    check-shadowing: false
+
+linters:
+  enable:
+    # mandatory linters
+    - govet
+    - golint
+
+    # some default golangci-lint linters
+    - deadcode
+    - errcheck
+    - gosimple
+    - ineffassign
+    - staticcheck
+    - structcheck
+    - typecheck
+    - unused
+    - varcheck
+
+    # extra linters
+    # - exhaustive
+    - gofmt
+    - whitespace
+    - goimports
+  disable-all: true
+  fast: false
+
+issues:
+  include:
+    - EXC0002 # should have a comment
+    - EXC0003 # test/Test ... consider calling this
+    - EXC0004 # govet
+    - EXC0005 # C-style breaks