docs: Add styleguide.

docs/Styleguide.md

+# Styleguide
+
+## Guidelines
+
+In general CBM follows the ALICE styleguide. It can be found here:
+
+- [Coding guidelines](https://rawgit.com/AliceO2Group/CodingGuidelines/master/coding_guidelines.html)
+- [Naming and formatting conventions](https://rawgit.com/AliceO2Group/CodingGuidelines/master/naming_formatting.html)
+- [Comments guidelines](https://rawgit.com/AliceO2Group/CodingGuidelines/master/comments_guidelines.html)
+
+## Exceptions
+
+Note that we've adopted some exceptions to these rules:
+
+### Header files
+
+Use `#pragma once` for header files instead of `#define` guards.
+
+_Reason_: Less error prone and doesn't have to be altered when the file path changes. Potential drawbacks (issues with symlinks, compiler support) don't seem relevant for CbmRoot.
+
+### Naming
+
+#### Function names
+
+Function names should start with a capital letter. (Instead of lower case in ALICE)
+
+_Reason_: Compatibility with old code.
+
+#### Class members
+
+Class members should be prefixed with f (instead of m in ALICE). (Struct members are never prefixed!)
+
+_Reason_: No advantage in changing prefix. Stay compatible with existing code.
+
+### Formatting
+
+Formatting differs in details from ALICE. Differences are caught with clang-format.