docs/review_comments.md: Start document with frequent review comments.

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+# Code Review Comments
+
+This is a collection of frequent code review comments, collected here for
+reference and discussed in more depth than a typical code review would allow.
+
+The intended use for this document is to point to it from a code review to make
+a point quickly while still providing the contributor with enough information
+to resolve the issue. For example, a good review comment would be:
+
+![Please initialize variables at declaration. Link to comment.](review_comments_example.png)
+
+A link to each paragraph is provided at the beginning for easy copy'n'pasting.
+
+## Initialize variables
+
+→ [https://collectd.org/review-comments#initialize-variables](https://collectd.org/review-comments#initialize-variables)
+
+Initialize variables when declaring them. By default, C does not initialize
+variables when they are declared. If a code path ends up reading the variable
+before it is initialized, for example because a loop body is never
+executed, it will read random data, causing undefined behavior. Worst case,
+pointers will point to random memory causing a segmentation fault.
+
+**Examples:**
+
+```c
+/* Initialize scalar with to literal: */
+int status = 0;
+
+/* Initialize pointer with function call: */
+char *buffer = calloc(1, buffer_size);
+
+/* Initialize struct with struct initializer: */
+struct addrinfo ai = {
+  .ai_family = AF_UNSPEC,
+  .ai_flags = AI_ADDRCONFIG,
+  .ai_socktype = SOCK_STREAM,
+};
+
+/* Initialize struct with zero: */
+struct stat statbuf = {0};
+```
+
+In the last example, `{0}` is the universal struct initializer that, in theory,
+should be able to zero-initialize any struct. In practise, however, some
+compilers don't implement this correctly and will get confused when the first
+member is a struct or a union. Our *continuous integration* framework will
+catch these cases.
+
+In many cases, this means declaring variables later. For example, this *bad*
+example:
+
+```c
+int status; /* BAD */
+
+/* other code */
+
+status = function_call();
+```
+
+Would become:
+
+```c
+/* other code */
+
+int status = function_call(); /* GOOD */
+```

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