Organize links in README

README.md

@@ -12,17 +12,28 @@ Table of contents
 
 * [Overview](#libshmemq)
 * [Table of contents](#table-of-contents)
-* [External links](#external-links)
+* [Links](#links)
+  * [Similar implementations](#similar-implementations)
+  * [Implementation theory](#implementation-theory)
+  * [Cost theory](#cost-theory)
 
 
 
-External links
---------------
+Links
+-----
+
+### Similar implementations
 
 * [goldshtn/shmemq-blog](https://github.com/goldshtn/shmemq-blog)
 * [MengRao/SPSC_Queue](https://github.com/MengRao/SPSC_Queue)
 * [ezhang887/shmemq](https://github.com/ezhang887/shmemq)
+
+### Implementation theory
+
 * [Building a shared memory IPC implementation – Part I](https://coherent-labs.com/posts/building-a-shared-memory-ipc-implementation-part-i/)
 * [Single-Producer/Single-Consumer Queue](https://software.intel.com/content/www/us/en/develop/articles/single-producer-single-consumer-queue.html)
+
+### Cost theory
+
 * [Which takes longer time? Switching between the user & kernel modes or switching between two processes?](https://stackoverflow.com/a/14205346)
 * [What happens the most, context switches or mode switches?](https://unix.stackexchange.com/a/15537)