docs: Update required Clang version and Ubuntu 14.04 instructions (#150)

[ci skip]

README.md

@@ -46,7 +46,7 @@ sudo emerge -a nheko
 - CMake 3.1 or greater.
 - [LMDB](https://symas.com/lightning-memory-mapped-database/).
 - A compiler that supports C++11.
-    - Clang 3.3 (or greater).
+    - Clang 3.6 (or greater).
     - GCC 4.9.4 (or greater).
 
 ##### Arch Linux
@@ -70,6 +70,15 @@ sudo apt-get update
 sudo apt-get install -y qt59base qt59tools qt59multimedia cmake liblmdb-dev
 ```
 
+To build on Ubuntu 14.04 Trusty out-of-the-box requires using Clang 3.6 instead of GCC:
+
+```bash
+sudo apt-get install clang-3.6
+export CC=clang-3.6 CXX=clang++-3.6
+```
+
+On Ubuntu 14.04 Trusty, it's possible to use GCC 4.9.4+, but it is not recommended, because it requires installing GCC packages from third-party PPAs.  Later versions of Ubuntu that come with GCC 4.9.4+ should work with GCC out-of-the-box.
+
 ##### OSX (Xcode 8 or later)
 
 ```bash