readme: add installation instructions

Problem: there are no instructions in the readme for
how to install flux-pmix.

Add some.

Author: wihobbs

README.md

@@ -21,6 +21,39 @@ or by setting the `FLUX_PMI_CLIENT_METHODS` environment variable globally:
 FLUX_PMI_CLIENT_METHODS="simple pmix single"
 ```
 
+### installation
+
+##### Building From Source
+
+```console
+./autogen.sh
+./configure --localstatedir=/var/
+make -j
+make check
+```
+
+To configure flux-pmix with a side-installed version of pmix and/or flux-core, 
+you will need to make sure that side install's pkg-config files are exported. 
+For an example of how to do that, see the example script below:
+
+```console
+#!/bin/bash
+PMIX_PREFIX=/opt/openpmix-4.2.2
+FLUX_CORE_PREFIX=/home/elvis/flux
+
+PKG_CONFIG_PATH=$(pkg-config --variable pc_path pkg-config)
+PKG_CONFIG_PATH=${PMIX_PREFIX}/lib/pkgconfig:${PKG_CONFIG_PATH}
+PKG_CONFIG_PATH=${FLUX_CORE_PREFIX}/lib/pkgconfig:${PKG_CONFIG_PATH}
+
+./autogen.sh
+PKG_CONFIG_PATH=${PKG_CONFIG_PATH} \
+     ./configure --prefix=${FLUX_CORE_PREFIX}
+make && make install
+```
+
+Run that before `make`.
+
+
 ### limitations
 
 The pmix specs cover a broad range of topics.  Although the shell plugin is