INSTALL.md: it's mind-boggling how bad Windows is and how hard it is to use

A few changes:
  * mention pre-requisites
  * provide a direct link to the raw git-filter-repo file
  * note that `git_filter_repo.py` and `git-filter-repo` are filenames
  * mention for the windows folks that the python executable can be a
    full absolute path as well.

Signed-off-by: Elijah Newren <newren@gmail.com>

INSTALL.md

@@ -1,5 +1,6 @@
 # Table of Contents
 
+  * [Pre-requisites](#pre-requisites)
   * [Simple Installation](#simple-installation)
   * [Installation via Package Manager](#installation-via-package-manager)
   * [Detailed installation explanation for
@@ -7,10 +8,18 @@
   * [Installation via Makefile](#installation-via-makefile)
   * [Notes for Windows Users](#notes-for-windows-users)
 
+# Pre-requisites
+
+Instructions on this page assume you have already installed both
+[Git](https://git-scm.com) and [Python](https://www.python.org/).
+
 # Simple Installation
 
-All you need to do is download the [git-filter-repo script in this
-repository](git-filter-repo).  **That's it**.  You're done.
+All you need to do is download one file: the [git-filter-repo script
+in this repository](git-filter-repo) ([direct link to raw
+file](https://raw.githubusercontent.com/newren/git-filter-repo/main/git-filter-repo)),
+making sure to preserve its name (`git-filter-repo`, with no
+extension).  **That's it**.  You're done.
 
 Then you can run any command you want, such as
 
@@ -25,8 +34,8 @@ installation hassle for you, or you really want some kind of "official
 installation", the other sections may have useful tips.
 
 Optionally, if you also want to use some of the contrib scripts, then
-you need to make sure you have a git_filter_repo.py file which is
-either a link to or copy of git-filter-repo, and you need to place
+you need to make sure you have a `git_filter_repo.py` file which is
+either a link to or copy of `git-filter-repo`, and you need to place
 that git_filter_repo.py file in $PYTHONPATH.
 
 # Installation via Package Manager
@@ -138,15 +147,17 @@ you'll still need to do that yourself.
 
 Windows likes to make things difficult.  Common and historical issues:
 
-  * [**Non-functional Python
-    stub**](https://github.com/newren/git-filter-repo/issues/36#issuecomment-568933825):
-    Windows apparently ships with a non-functional python.  This can
-    even manifest as [the app
+  * **Non-functional Python stub**: Windows apparently ships with a
+    [non-functional
+    python](https://github.com/newren/git-filter-repo/issues/36#issuecomment-568933825).
+    This can even manifest as [the app
     hanging](https://github.com/newren/git-filter-repo/issues/36) or
     [the system appearing to
     hang](https://github.com/newren/git-filter-repo/issues/312).  Try
-    installing Python from the [Microsoft
-    Store](https://docs.microsoft.com/en-us/windows/python/beginners)
+    installing
+    [Python](https://docs.microsoft.com/en-us/windows/python/beginners)
+    from the [Microsoft
+    Store](https://apps.microsoft.com/store/search?publisher=Python%20Software%20Foundation)
   * **Modifying PATH, making the script executable**: For some reason lots of
     Windows users have a hard time modifying their PATH and/or making scripts
     executable.  You can skip that step by just using
@@ -159,7 +170,7 @@ Windows likes to make things difficult.  Common and historical issues:
     filter-repo commands.  Namely, replace the `git filter-repo` part
     of commands with `PYTHON_EXECUTABLE git-filter-repo`. (Where
     `PYTHON_EXECUTABLE` is something like `python` or `python3.8` or
-    whatever).
+    `C:\PATH\TO\INSTALLATION\OF\python3.exe` or whatever).
   * **Symlink issues**:  git_filter_repo.py is supposed to be a symlink to
     git-filter-repo, so that it appears to have identical contents.
     If your system messed up the symlink (usually meaning it looks like a