Created a C Style Guide for upb.

PiperOrigin-RevId: 447364819

docs/style-guide.md

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+# C style guide
+
+<!--*
+# Document freshness: For more information, see go/fresh-source.
+freshness: { owner: 'haberman' reviewed: '2022-05-08' }
+*-->
+
+Since upb is written in pure C, we supplement the
+[Google C++ style guide](https://google.github.io/styleguide/cppguide.html) with
+some C-specific guidance.
+
+Everything written here is intended to follow the spirit of the C++ style guide.
+
+upb is currently inconsistent about following these conventions. It is intended
+that all code will be updated to match these guidelines. The priority is
+converting public interfaces as these are more difficult to change later.
+
+## Naming
+
+### Functions and Types
+
+C does not have namespaces. Anywhere you would normally use a namespace
+separator (`::`) in C++, we use an underscore (`_`) in C:
+
+```c++
+// C equivalent for upb::Arena::New()
+upb_Arena* upb_Arena_New();
+```
+
+Since we rely on `_` to be our namespace separator, we never use it to merely
+separate words in function or type names:
+
+```c++
+// BAD: this would be interpreted as upb::FieldDef::has::default().
+bool upb_FieldDef_has_default(const upb_FieldDef* f);
+
+// GOOD: this is equivalent to upb::FieldDef::HasDefault().
+bool upb_FieldDef_HasDefault(const upb_FieldDef* f);
+```
+
+For multi-word namespaces, we use `PascalCase`:
+
+```c++
+// `PyUpb` is the namespace.
+PyObject* PyUpb_CMessage_GetAttr(PyObject* _self, PyObject* attr);
+```
+
+### Private Functions and Members
+
+Since we do not have `private` in C++, we use a leading underscore convention
+to mark internal functions and variables that should only be accessed from
+upb:
+
+```c++
+// Internal-only function.
+int64_t _upb_Int64_FromLL();
+
+// Internal-only members. Underscore prefixes are only necessary when the
+// structure is defined in a header file.
+typedef struct {
+  const int32_t* _values;  // List of values <0 or >63
+  uint64_t _mask;          // Bits are set for acceptable value 0 <= x < 64
+  int _value_count;
+} upb_MiniTable_Enum;
+```