Add a doc for contributor tips (#2480)

Contains some recommendations for tools and git setup for people
contributing to IREE. This is an opinionated guide that is largely just
*my* setup, but I know it overlaps significantly with other people's as
well and hopefully it can evolve into something more general.
diff --git a/scripts/git/bazel_to_cmake.sh b/scripts/git/bazel_to_cmake.sh
index 5b8234f..e5738fa 100755
--- a/scripts/git/bazel_to_cmake.sh
+++ b/scripts/git/bazel_to_cmake.sh
@@ -19,7 +19,7 @@
 #
 # - Requries the gh CLI (https://github.com/cli/cli) to create a PR.
 # - Will force push to the configured PR_BRANCH (default "bazel-to-cmake-fix")
-#   on the configured FORK_REMOTE (default "fork")
+#   on the configured FORK_REMOTE (default "origin")
 # - Requires that local BASE_BRANCH branch is a pristine (potentially stale)
 #   copy of the same branch on the configured UPSTREAM_REMOTE
 #   (default "upstream").
@@ -32,7 +32,7 @@
 BASE_BRANCH="${1:-google}"
 PR_BRANCH="bazel-to-cmake-fix"
 UPSTREAM_REMOTE="${UPSTREAM_REMOTE:-upstream}"
-FORK_REMOTE="${FORK_REMOTE:-fork}"
+FORK_REMOTE="${FORK_REMOTE:-origin}"
 
 if [[ -n "$(git status --porcelain)" ]]; then
   echo "Working directory not clean. Aborting"
diff --git a/scripts/git/google_to_main.sh b/scripts/git/google_to_main.sh
index bb64413..0ec91a7 100755
--- a/scripts/git/google_to_main.sh
+++ b/scripts/git/google_to_main.sh
@@ -19,7 +19,7 @@
 #
 # - Requries the gh CLI (https://github.com/cli/cli) to create a PR.
 # - Will force push to the configured PR_BRANCH (default "google-to-main") on
-#   the configured FORK_REMOTE (default "fork")
+#   the configured FORK_REMOTE (default "origin")
 # - Requires that local "google" branch is a pristine (potentially stale) copy
 #   of the "google" branch on the configured UPSTREAM_REMOTE
 #   (default "upstream").
@@ -33,7 +33,7 @@
 
 PR_BRANCH="${PR_BRANCH:-google-to-main}"
 UPSTREAM_REMOTE="${UPSTREAM_REMOTE:-upstream}"
-FORK_REMOTE="${FORK_REMOTE:-fork}"
+FORK_REMOTE="${FORK_REMOTE:-origin}"
 
 if [[ -n "$(git status --porcelain)" ]]; then
   echo "Working directory not clean. Aborting"
diff --git a/scripts/git/main_to_google.sh b/scripts/git/main_to_google.sh
index 4de198a..53c2513 100755
--- a/scripts/git/main_to_google.sh
+++ b/scripts/git/main_to_google.sh
@@ -19,7 +19,7 @@
 #
 # - Requries the gh CLI (https://github.com/cli/cli) to create a PR.
 # - Will force push to the configured PR_BRANCH (default "main-to-google") on
-#   the configured FORK_REMOTE (default "fork")
+#   the configured FORK_REMOTE (default "origin")
 # - Requires that local "main" branch is a pristine (potentially stale) copy
 #   of the "main" branch on the configured UPSTREAM_REMOTE
 #   (default "upstream").
@@ -33,7 +33,7 @@
 
 PR_BRANCH="${PR_BRANCH:-main-to-google}"
 UPSTREAM_REMOTE="${UPSTREAM_REMOTE:-upstream}"
-FORK_REMOTE="${FORK_REMOTE:-fork}"
+FORK_REMOTE="${FORK_REMOTE:-origin}"
 
 if [[ -n "$(git status --porcelain)" ]]; then
   echo "Working directory not clean. Aborting"
diff --git a/scripts/git/update_tf_submodule.sh b/scripts/git/update_tf_submodule.sh
index 0d6f776..a3aac63 100755
--- a/scripts/git/update_tf_submodule.sh
+++ b/scripts/git/update_tf_submodule.sh
@@ -20,7 +20,7 @@
 #
 # - Requries the gh CLI (https://github.com/cli/cli) to create a PR.
 # - Will force push to the configured PR_BRANCH (default "tf-submodule-update")
-#   on the configured FORK_REMOTE (default "fork")
+#   on the configured FORK_REMOTE (default "origin")
 # - Requires that local BASE_BRANCH branch is a pristine (potentially stale)
 #   copy of the same branch on the configured UPSTREAM_REMOTE
 #   (default "upstream").
@@ -37,7 +37,7 @@
 PR_BRANCH="tf-submodule-update"
 BASE_BRANCH="${1:-google}"
 UPSTREAM_REMOTE="${UPSTREAM_REMOTE:-upstream}"
-FORK_REMOTE="${FORK_REMOTE:-fork}"
+FORK_REMOTE="${FORK_REMOTE:-origin}"
 TF_COMMIT_NICKNAME=""
 
 if [[ -n "$(git status --porcelain)" ]]; then
@@ -82,5 +82,5 @@
 )"
 
 git commit -am "${TITLE?}"
-git push -f fork "${PR_BRANCH?}"
+git push -f "${FORK_REMOTE?}" "${PR_BRANCH?}"
 gh pr create --title="${TITLE?}" --body="${BODY?}" --base="${BASE_BRANCH?}"