README: merge build systems and toolchains list

foxpy · MBkkt · commit a054c03acf8f · 2022-03-03T12:31:29.000+03:00
Previously, user might erroneously think he doesn't need to
install gcc, g++ and clang, but that's not true. For clarity sake,
I am merging these into a single category "Need to install".
diff --git a/README.md b/README.md
@@ -14,10 +14,6 @@ Need to install:
 ```
 cmake
 ninja
-```
-
-Also you can install:
-```
 gcc
 g++
 clang

-Original file line number
+Diff line change
 ```
 cmake
 ninja
 -```
+-
 -Also you can install:
 -```
 gcc
 g++
 clang