Update INSTALL.md to ensure AVX2 and AV512 support for python package (…

…facebookresearch#3892)

Summary:
Following the current documentation creates the python package without AVX2 or AV512 support. Updated documentation notes that corresponding faiss version must be built before swigfaiss.

fixes facebookresearch#3883

Pull Request resolved: facebookresearch#3892

Reviewed By: mengdilin

Differential Revision: D63641111

Pulled By: asadoughi

fbshipit-source-id: 2f0598ead8cc5b82ed34841c185e6d2a1d068ba5

INSTALL.md

@@ -167,6 +167,22 @@ The `-j` option enables parallel compilation of multiple units, leading to a
 faster build, but increasing the chances of running out of memory, in which case
 it is recommended to set the `-j` option to a fixed value (such as `-j4`).
 
+If making use of optimization options, build the correct target before swigfaiss.
+
+For AVX2:
+
+``` shell
+$ make -C build -j faiss_avx2
+```
+
+For AVX512:
+
+``` shell
+$ make -C build -j faiss_avx512
+```
+
+This will ensure the creation of neccesary files when building and installing the python package.
+
 ## Step 3: Building the python bindings (optional)
 
 ``` shell
@@ -177,6 +193,7 @@ $ (cd build/faiss/python && python setup.py install)
 The first command builds the python bindings for Faiss, while the second one
 generates and installs the python package.
 
+
 ## Step 4: Installing the C++ library and headers (optional)
 
 ``` shell