Add command to download cpu_features submodule

Author: nicodr97

docs/source/installation.rst

@@ -56,44 +56,49 @@ The simplest way to compile this package is:
 
      $ cd trimal
 
-  2. Create a custom makefile. The CMake build will autodetect the best CPU features available. They can be individually disabled:
+  2. Run this to clone the `cpu_features` submodule:
+     ::
+
+     $ git submodule update --init
+
+  3. Create a custom makefile. The CMake build will autodetect the best CPU features available. They can be individually disabled:
 
-     2.1. Build with AVX2, SSE2 and NEON (and effectively use AVX2 on x86 and NEON in arm)
+     3.1. Build with AVX2, SSE2 and NEON (and effectively use AVX2 on x86 and NEON in arm)
 
      ::
 
      $ cmake . 
 
-     2.2. Build with SSE2 only
+     3.2. Build with SSE2 only
 
      ::
 
      $ cmake . -DDISABLE_AVX2=1 -DDISABLE_NEON=1
 
-     2.3. Build with AVX2 only
+     3.3. Build with AVX2 only
 
      ::
 
      $ cmake . -DDISABLE_SSE2=1 -DDISABLE_NEON=1
 
-     2.4. Build with NEON only
+     3.4. Build with NEON only
 
      ::
 
      $ cmake . -DDISABLE_SSE2=1 -DDISABLE_AVX2=1
 
-     2.5. Build without SIMD
+     3.5. Build without SIMD
 
      ::
 
      $ cmake . -DDISABLE_SSE2=1 -DDISABLE_AVX2=1 -DDISABLE_NEON=1
 
-  3. Compile the package.
+  4. Compile the package.
      ::
 
      $ make
 
-  4. Optionally, run trimAl/readAl with the examples into the 'dataset' 
+  5. Optionally, run trimAl/readAl with the examples into the 'dataset' 
      directory to check the correct installation. It should return the original MSA.
      ::