meta: add clang-format formatting configuration

This allows for voluntary formatting with clang-format (or
IDE tools like clang-format plugin for VS Code).

Once a suitable formatting configuration is found, automatic formatting
can be configured

Tweaked by Jakub Jelen

.clang-format

@@ -0,0 +1,42 @@
+BasedOnStyle: LLVM
+# defaults from LLVM
+# BreakBeforeBraces: Attach
+# AllowShortIfStatementsOnASingleLine: Never
+# IndentCaseLabels: false
+# AlignAfterOpenBracket: Align
+# AlignTrailingComments: true
+# MaxEmptyLinesToKeep: 1
+# ReflowComments: falsedefault
+# SortIncludes: true
+# BreakBeforeBinaryOperators: None
+# BraceWrapping:
+#   AfterClass:      false
+#   AfterControlStatement: false
+#   AfterFunction:   false
+#   AfterNamespace:  false
+#   BeforeCatch:     false
+#   BeforeElse:      false
+#   IndentBraces:    false
+
+# OpenSC modifications
+TabWidth: 8
+IndentWidth: 8
+ContinuationIndentWidth: 16
+AlignAfterOpenBracket: DontAlign
+UseTab: Always
+AlignConsecutiveMacros: true
+AlignEscapedNewlines: DontAlign
+AllowShortFunctionsOnASingleLine: None
+AlwaysBreakAfterReturnType: AllDefinitions
+## This prevents reflowing intentionally short lines but
+##  it can be allowed only after we will have some baseline
+ColumnLimit: 0
+#ColumnLimit: 110
+IndentCaseBlocks: false
+AlignArrayOfStructures: Left
+
+BreakBeforeBraces: Custom
+BraceWrapping:
+  AfterObjCDeclaration: true
+  AfterUnion:      true
+  AfterFunction:   true

CONTRIBUTING.md

@@ -1,3 +1,45 @@
+# Formatting style
+
+The OpenSC formatting rules are described in `.clang-format` in the root
+directory. It is based on [LLVM](https://llvm.org/docs/CodingStandards.html)
+style with couple of modifications:
+
+ * Tabs
+   * Tabs are used instead of spaces
+   * Tab is 8 spaces wide
+ * The maximum line width is 110 characters
+ * Opening braces follow the condition/expression except for the functions
+
+Examples:
+
+```
+void
+function_name(int arg)
+{
+	int var = 0;
+	int rc = 0;
+
+	if (arg) {
+		var = do_something();
+	}
+	if (rc = call_some_function(arg) ||
+			rc = call_some_other_long_funct(arg) ||
+			rc = call_one_more_func(arg)) {
+		/* Note the two Tabs on the line above ! */
+		return rc;
+	}
+	return var;
+}
+```
+
+To check your changes if they follow the formatting style (before submitting
+a PR), you can use `clang-format` tool or `git-clang-format`, which can check
+only the parts of the code you changed in your branch
+
+```
+$ git-clang-format --diff --commit upstream/master
+```
+
 # Testing locally
 
 To learn how to run the tests from Github actions locally in containers, see