add docs and some hashmap code

notes/advanced-techniques.md

@@ -85,5 +85,3 @@ union epoll_data {
     uint64_t  u64;
 };
 ```
-
-**TODO: improve the notes for epoll()**

notes/hash-table.md

@@ -17,5 +17,3 @@ $$ hash(\text{key}) = \text{key} \mod \text{table\_dim} $$
 
 - String keys:
 $$ hash(key) = \sum_{i=0}^{len(key) - 1} ascii\_value(key[i]) * prime\_number$$
-
-**TODO**