README.md: add basic installation instructions for Ubuntu

Note: these instructions were first posted here:
https://github.com/rtyley/bfg-repo-cleaner/issues/255#issuecomment-1500711841

Signed-off-by: Gabriel Staples <ercaguy@gmail.com>

README.md

@@ -46,7 +46,59 @@ filter-repo requires:
 
 `git-filter-repo` is a single-file python script, which was done to make
 installation for basic use on many systems trivial: just place that
-file into your $PATH.
+file into your `$PATH`. Example:
+
+### How to install `git filter-repo` on Linux Ubuntu:
+
+```bash
+# --------------------
+# 1. Download it
+# --------------------
+
+# Option 1 [easiest]: download the latest version
+wget https://raw.githubusercontent.com/newren/git-filter-repo/main/git-filter-repo
+
+# OR Option 2: get the latest *released* version.
+# Go here and find the latest release:
+# https://github.com/newren/git-filter-repo/releases
+# Use that release number in the cmds below.
+wget https://github.com/newren/git-filter-repo/releases/download/v2.38.0/git-filter-repo-2.38.0.tar.xz
+# install dependencies
+sudo apt install xz-utils  
+# extract the downloaded archive
+tar -xf git-filter-repo-2.38.0.tar.xz
+# copy out the executable; we'll move it to a directory within our PATH later
+cp git-filter-repo-2.38.0/git-filter-repo .
+
+# --------------------
+# 2. make it executable
+# --------------------
+chmod +x git-filter-repo
+
+# --------------------
+# 3. move it to a dir in your PATH
+# --------------------
+
+# Option 1 [easiest]: make this executable accessible to ALL users
+sudo mv -i git-filter-repo /usr/local/bin
+
+# OR Option 2: make this executable accessible to your user only
+mkdir -p ~/bin
+mv -i git-filter-repo ~/bin 
+# add ~/bin to your path by re-sourcing your `~/.profile` file.
+# This works on Ubuntu if you are using the default ~/.profile file, which can
+# also be found in /etc/skel/.profile, by the way.
+. ~/.profile
+
+# --------------------
+# 4. Done. Now run it.
+# --------------------
+git filter-repo --version       # check the version
+git filter-repo -h              # help menu
+git filter-repo -h | less -RFX  # help menu, viewed in the `less` viewer
+```
+
+### Advanced install
 
 See [INSTALL.md](INSTALL.md) for things beyond basic usage or special
 cases.  The more involved instructions are only needed if one of the